Adds automatic window updates

Implements the first two items in the feature list of #27

- [x] add window_update method to both Stream and Connection
- [x] automatic WU after emit(:data)

lib/http/2/connection.rb

@@ -154,6 +154,11 @@ def settings(payload)
       @pending_settings << payload
     end
 
+    # Sends a WINDOW_UPDATE frame
+    def window_update(increment)
+      send(type: :window_update, stream: 0, increment: increment)
+    end
+
     # Decodes incoming bytes into HTTP 2.0 frames and routes them to
     # appropriate receivers: connection frames are handled directly, and
     # stream frames are passed to appropriate stream objects.
@@ -626,6 +631,7 @@ def activate_stream(id: nil, **args)
       stream.once(:close)  { @active_stream_count -= 1 }
       stream.on(:promise, &method(:promise)) if self.is_a? Server
       stream.on(:frame,   &method(:send))
+      stream.on(:window_update, &method(:window_update))
 
       @streams[id] = stream
     end

lib/http/2/stream.rb

@@ -97,8 +97,14 @@ def receive(frame)
 
       case frame[:type]
       when :data
-        @local_window -= frame[:payload].size
+        @local_window -= frame[:payload].bytesize
         emit(:data, frame[:payload]) unless frame[:ignore]
+
+        # Automatically send a window_update with the size of
+        # the received bytes as the increment
+        if frame[:payload] && frame[:payload].bytesize > 0
+          window_update(frame[:payload].bytesize)
+        end
       when :headers, :push_promise
         emit(:headers, frame[:payload]) unless frame[:ignore]
       when :priority
@@ -206,6 +212,14 @@ def refuse
       send(type: :rst_stream, error: :refused_stream)
     end
 
+    # Sends a WINDOW_UPDATE frame if the stream is in the correct state
+    # for doing so
+    def window_update(increment)
+      emit(:window_update, increment)  # alway allow the connection update
+      return if @state == :closed || @state == :remote_closed
+      send(type: :window_update, increment: increment)
+    end
+
     private
 
     # HTTP 2.0 Stream States