Closure of stream with pending frames causes GO_AWAY.

Motivation:

The current logic in DefaultHttp2OutboundFlowController for handling the
case of a stream shutdown results in a Http2Exception (not a
Http2StreamException). This results in a GO_AWAY being sent for what
really could just be a stream-specific error.

Modifications:

Modified DefaultHttp2OutboundFlowController to set a stream exception
rather than a connection-wide exception.  Also using the error code of
INTERNAL_ERROR rather than STREAM_CLOSED, since it's more appropriate
for this case.

Result:

Should not be triggering GO_AWAY when a stream closes prematurely.