Stopped sending empty frame on stream initiated in sender half-closed state.

The SPDY protocol states the sender of FLAG_FIN must not send further frames
on that stream.  This change skips the sending of a final (empty) frame with
FLAG_FIN set when the stream is already half-closed because SYN_STREAM was
sent with FLAG_FIN set.

okhttp/src/main/java/com/squareup/okhttp/internal/spdy/SpdyStream.java

@@ -670,7 +670,9 @@ public final class SpdyStream {
         }
         closed = true;
       }
-      writeFrame(true);
+      if (!out.finished) {
+        writeFrame(true);
+      }
       connection.flush();
       cancelStreamIfNecessary();
     }