add test case for server sending GOAWAY before opening a stream

[jprjr]

Hi there - this is for issue #179 - it just adds a test case for what (I think) the expected behavior is - if a connection has received a GOAWAY frame before a stream is opened, then opening a stream should result in an error.

Happy to change if the expectation is elsewhere (that writing headers should be an error, or receiving should be an error)