Add a Stream.gettrailers() method.

This is needed so that we can lazily load trailers into the
HTTP20Response object. We don't want to force ourselves to download
every stream completely just in case it has trailers, but if we're asked
for them we need to be able to get them.

Author: Lukasa

hyper/http20/stream.py

@@ -243,7 +243,6 @@ def receive_frame(self, frame):
             # We've handled the headers, zero them out.
             self.header_data = None
 
-
     def open(self, end):
         """
         Open the stream. Does this by encoding and sending the headers: no more
@@ -300,6 +299,28 @@ def getheaders(self):
 
         return self.response_headers
 
+    def gettrailers(self):
+        """
+        Once all data has been sent on this connection, returns a key-value set
+        of the trailers of the response to the original request.
+
+        .. warning:: Note that this method requires that the stream is
+                     totally exhausted. This means that, if you have not
+                     completely read from the stream, all stream data will be
+                     read into memory.
+
+        :returns: The key-value set of the trailers, or ``None`` if no trailers
+                  were sent.
+        """
+        # Keep reading until the stream is done.
+        # TODO: Right now this doesn't handle CONTINUATION blocks in trailers.
+        # The idea of receiving such a thing is mind-boggling it's so unlikely,
+        # but we should fix this up at some stage.
+        while not self._remote_closed:
+            self._recv_cb()
+
+        return self.response_trailers
+
     def getpushes(self, capture_all=False):
         """
         Returns a generator that yields push promises from the server. Note that