&HeaderValues implements ToHeaderValues

one of the surf examples included lines like this:
```
for (name, value) in &req {
  new_req.insert_header(name.clone(), &value[..]);
}
```

and it seemed like it would be nice to do

```
for (name, value) in &req {
  new_req.insert_header(name, value);
}
```

Author: jbr

src/headers/to_header_values.rs

@@ -3,7 +3,7 @@ use std::iter;
 use std::option;
 use std::slice;
 
-use crate::headers::HeaderValue;
+use crate::headers::{HeaderValue, HeaderValues, Values};
 
 /// A trait for objects which can be converted or resolved to one or more `HeaderValue`s.
 pub trait ToHeaderValues {
@@ -22,6 +22,14 @@ impl ToHeaderValues for HeaderValue {
     }
 }
 
+impl<'a> ToHeaderValues for &'a HeaderValues {
+    type Iter = iter::Cloned<Values<'a>>;
+
+    fn to_header_values(&self) -> crate::Result<Self::Iter> {
+        Ok(self.iter().cloned())
+    }
+}
+
 impl<'a> ToHeaderValues for &'a [HeaderValue] {
     type Iter = iter::Cloned<slice::Iter<'a, HeaderValue>>;