response: clarify Content-Disposition, close file in Stream example (#351)

It wasn't clear to me what were the differences between `Context#File`,
`Context#Attachment` and `Context#Inline` - reading the source made it
clearer: the former 2 are referring to sending `Content-Disposition`
with `attachment` and `inline`, which makes sense.

Adding missing `defer f.Close()` in the `Context#Stream` example. I also
checked, it takes an `io.Reader` not an `io.ReadCloser` so it couldn't
delegate the close to `Context#Stream`.

Author: StalkR

website/docs/guide/response.md

@@ -244,7 +244,7 @@ func(c echo.Context) error {
 ## Send Attachment
 
 `Context#Attachment(file, name string)` is similar to `File()` except that it is
-used to send file as attachment with provided name.
+used to send file as `Content-Disposition: attachment` with provided name.
 
 *Example*
 
@@ -257,7 +257,7 @@ func(c echo.Context) error {
 ## Send Inline
 
 `Context#Inline(file, name string)` is similar to `File()` except that it is
-used to send file as inline with provided name.
+used to send file as `Content-Disposition: inline` with provided name.
 
 *Example*
 
@@ -296,6 +296,7 @@ func(c echo.Context) error {
   if err != nil {
     return err
   }
+  defer f.Close()
   return c.Stream(http.StatusOK, "image/png", f)
 }
 ```