Add `html_utf8` content type.

src/http/header/content_type.rs

@@ -80,6 +80,13 @@ impl ContentType {
         ContentType(mime::TEXT_HTML)
     }
 
+    /// A constructor to easily create a `Content-Type: text/html; charset=utf-8`
+    /// header.
+    #[inline]
+    pub fn html_utf8() -> ContentType {
+        ContentType(mime::TEXT_HTML_UTF_8)
+    }
+
     /// A constructor to easily create a `Content-Type: text/xml` header.
     #[inline]
     pub fn xml() -> ContentType {