Fix flaky TestStreamSendError
Fix flaky TestStreamSendError

Test was seeing "connection reset by peer" errors. This occurs when the
remote side sends a RST. The sequence of events that led to this:

 - client sends "send me an error" message
 - server sends error message, does not close the reader.
 - client reads error message
 - client then calls "Close" on the argWriter, which causes
   callReqContinue to be sent
 - test function ends, Closing the server
 - server receives the callReqContinue on a now closed TCP connection,
   and sends a RST packet to force close the connection
 - client receives an error instead of EOF on the connection

The fix is simple: instead of flushing the "send me an error" message,
call Close which flushes the message to the other side.

uber/tchannel-go,uber/tchannel-go