fix(headers): remove charset from ContentType::json() convenience method

since the JSON mime type does not accept a charset, this patch removes
it

closes https://github.com/hyperium/hyper/issues/737

src/header/common/content_type.rs

@@ -61,7 +61,7 @@ impl ContentType {
     /// A constructor  to easily create a `Content-Type: application/json; charset=utf-8` header.
     #[inline]
     pub fn json() -> ContentType {
-        ContentType(mime!(Application/Json; Charset=Utf8))
+        ContentType(mime!(Application/Json))
     }
 
     /// A constructor  to easily create a `Content-Type: text/plain; charset=utf-8` header.