fix(headers): remove charset from ContentType::json() convenience m…

…ethod

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

closes #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.