Add client support for server push (#314)

This patch exposes push promises to the client API.

Closes #252

src/frame/headers.rs

@@ -380,6 +380,13 @@ impl PushPromise {
     }
 }
 
+impl PushPromise {
+    /// Consume `self`, returning the parts of the frame
+    pub fn into_parts(self) -> (Pseudo, HeaderMap) {
+        (self.header_block.pseudo, self.header_block.fields)
+    }
+}
+
 #[cfg(feature = "unstable")]
 impl PushPromise {
     pub fn new(
@@ -400,10 +407,6 @@ impl PushPromise {
         }
     }
 
-    pub fn into_parts(self) -> (Pseudo, HeaderMap) {
-        (self.header_block.pseudo, self.header_block.fields)
-    }
-
     pub fn fields(&self) -> &HeaderMap {
         &self.header_block.fields
     }