433262 - WebSocket / Advanced close use cases

+ ClientCloseTest implementation of various outlined use cases.
433262 - WebSocket / Advanced close use cases

+ Fixing bad assumptions in unit tests where the client would send a few
  frames to test the protocol behavior followed by the close frame.
  But the test expected the server to initiate the close, but this setup
  of the tests would mean that the client initiated the close.
Allow subclasses to override the proxied request content.
Merged branch 'jetty-9.1.x' into 'master'.