Validate & convert messages before buffering

Malformed requests and responses should immediately result in a
RST_STREAM. To support this, received header frames are validated and
converted to Request / Response values immediately on receipt and before
buffering.

src/frame/mod.rs

@@ -73,16 +73,6 @@ pub enum Frame<T = Bytes> {
 }
 
 impl<T> Frame<T> {
-    /// Returns true if the frame is a DATA frame.
-    pub fn is_data(&self) -> bool {
-        use self::Frame::*;
-
-        match *self {
-            Data(..) => true,
-            _ => false,
-        }
-    }
-
     pub fn map<F, U>(self, f: F) -> Frame<U>
         where F: FnOnce(T) -> U
     {