transport: fix race causing flow control discrepancy when sending messages over server limit (#1859)

* In case of an error write transport quota back.

* Added test.

transport/http2_server.go

@@ -888,6 +888,8 @@ func (t *http2Server) Write(s *Stream, hdr []byte, data []byte, opts *Options) e
 			}
 			ltq, _, err := t.localSendQuota.get(size, s.waiters)
 			if err != nil {
+				// Add the acquired quota back to transport.
+				t.sendQuotaPool.add(tq)
 				return err
 			}
 			// even if ltq is smaller than size we don't adjust size since,