Return header in Stream.Header() if available

[ilonajulczuk]

Add additional nonblocking select clause that would
check if the header is available and return it if it is.

Current implementation of transport.go Stream Header often
doesn't return header, but a cancel error even though
the top level context is not canceled and the rpc
is successful and a header is actually available.
It's caused by the fact that select can choose any
"ready" channel.
Retrieving headers is flaky. With this fix, headers
are consistently retrieved for successful rpcs.

[ilonajulczuk]

Hi, could someone help me debugging the tests failure?
Travis doesn't provide any helpful message:
Also it only fails for the go 1.6.

When I test the code myself with "make" the tests pass and the command returns 0 status code.

[MakMukhi]

Finding the failed test on travis is tough in general. I usually search for "FAIL:". In your case this test failed:
FAIL: TestKeepaliveServerEnforcementWithAbusiveClientWithRPC (2.00s)
transport_test.go:694: Test failed: Expected a GoAway from server.

[menghanl]

It seems the test is flaky.
I restarted the test, and it passed.
Filed issue #1283 to track the flaky test.

[dfawley]

This doesn't deal with the goAway error. How about this version?

var err error
select {
case <-s.ctx.Done():
	err = ContextErr(s.ctx.Err())
case <-s.goAway:
	err = ErrStreamDrain
case <-s.headerChan:
	return s.header.Copy(), nil
}
// Even if the stream is closed, header is returned if available.
select {
case <-s.headerChan:
	return s.header.Copy(), nil
default:
}
return nil, err

[menghanl]

Thanks for the change. LGTM.