synchronise sending streamHeader

[sukunrt]

There is a race when sending stream header between Write and Close calls on sendStream

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[marten-seemann]

You don't need this check any more, do you?

[sukunrt]

I don't. Removed it.

[marten-seemann]

Test case seems to be flaky:

=== RUN   TestWriteCloseRace
    webtransport_test.go:88: 
        	Error Trace:	/Users/runner/work/webtransport-go/webtransport-go/webtransport_test.go:88
        	            				/Users/runner/work/webtransport-go/webtransport-go/webtransport_test.go:583
        	Error:      	Received unexpected error:
        	            	timeout: no recent network activity
        	Test:       	TestWriteCloseRace