provide workaround to save everyone that is using an external library that stored the http.Request as a map key.

eg. nosurf, gorilla context ..........

boy once people start using the built in context object on the http.Request they are going to find some fun quirks, because of the way context is tied to the http.Request
the http.Request changes and therefore libraries like nosurf store the http.Request in a map key, but as soon you store a value the http.Request changes and then the http.Request
cannot be found in the map anymore....

I can just imagine the number of applications that could break because of this.... anyway the temporary workaround will save you, but if you're using
a non `lars.Context` handler be sure to shallow copy the request to avoid this issue.

```go
// because 'r' is a copy of a pointer to allow the information to get
// back to the caller, need to set the value of 'r' as below with '*r'
func(w http.ResponseWriter, r *http.Request) {
    *r = *r.WithContext(context.WithValue(r.Context(), 0, "testval1"))
}
```

Author: Dean Karn

README.md

@@ -1,6 +1,6 @@
 ##LARS
 <img align="right" src="https://raw.githubusercontent.com/go-playground/lars/master/examples/README/test.gif">
-![Project status](https://img.shields.io/badge/version-3.3.0-green.svg)
+![Project status](https://img.shields.io/badge/version-3.3.1-green.svg)
 [![Build Status](https://semaphoreci.com/api/v1/projects/4351aa2d-2f94-40be-a6ef-85c248490378/679708/badge.svg)](https://semaphoreci.com/joeybloggs/lars)
 [![Coverage Status](https://coveralls.io/repos/github/go-playground/lars/badge.svg?branch=master)](https://coveralls.io/github/go-playground/lars?branch=master)
 [![Go Report Card](https://goreportcard.com/badge/go-playground/lars)](https://goreportcard.com/report/go-playground/lars)

context_17.go

@@ -79,7 +79,7 @@ func (c *Ctx) RequestStart(w http.ResponseWriter, r *http.Request) {
 // golang.org/x/net/context contained on this Context.
 // It is a shortcut for context.WithValue(..., ...)
 func (c *Ctx) Set(key interface{}, value interface{}) {
-	c.request = c.request.WithContext(context.WithValue(c.request.Context(), key, value))
+	*c.request = *c.request.WithContext(context.WithValue(c.request.Context(), key, value)) // temporarily shallow copying to avoid problems with external libraries
 }
 
 // Get returns the value for the given key and is a shortcut
@@ -104,7 +104,7 @@ func (c *Ctx) Context() context.Context {
 // WithContext updates the underlying request's context with to ctx
 // The provided ctx must be non-nil.
 func (c *Ctx) WithContext(ctx context.Context) {
-	c.request = c.request.WithContext(ctx)
+	*c.request = *c.request.WithContext(ctx) // temporarily shallow copying to avoid problems with external libraries
 }
 
 // Deadline calls the underlying golang.org/x/net/context Deadline()
@@ -131,29 +131,29 @@ func (c *Ctx) Value(key interface{}) interface{} {
 // updates context on the containing las.Context object.
 func (c *Ctx) WithCancel() context.CancelFunc {
 	ctx, cf := context.WithCancel(c.request.Context())
-	c.request = c.request.WithContext(ctx)
+	*c.request = *c.request.WithContext(ctx) // temporarily shallow copying to avoid problems with external libraries
 	return cf
 }
 
 // WithDeadline calls golang.org/x/net/context WithDeadline and automatically
 // updates context on the containing las.Context object.
 func (c *Ctx) WithDeadline(deadline time.Time) context.CancelFunc {
 	ctx, cf := context.WithDeadline(c.request.Context(), deadline)
-	c.request = c.request.WithContext(ctx)
+	*c.request = *c.request.WithContext(ctx) // temporarily shallow copying to avoid problems with external libraries
 	return cf
 }
 
 // WithTimeout calls golang.org/x/net/context WithTimeout and automatically
 // updates context on the containing las.Context object.
 func (c *Ctx) WithTimeout(timeout time.Duration) context.CancelFunc {
 	ctx, cf := context.WithTimeout(c.request.Context(), timeout)
-	c.request = c.request.WithContext(ctx)
+	*c.request = *c.request.WithContext(ctx) // temporarily shallow copying to avoid problems with external libraries
 	return cf
 }
 
 // WithValue calls golang.org/x/net/context WithValue and automatically
 // updates context on the containing las.Context object.
 // Can also use Set() function on Context object (Recommended)
 func (c *Ctx) WithValue(key interface{}, val interface{}) {
-	c.request = c.request.WithContext(context.WithValue(c.request.Context(), key, val))
+	*c.request = *c.request.WithContext(context.WithValue(c.request.Context(), key, val)) // temporarily shallow copying to avoid problems with external libraries
 }