servlet: extract ServletServerStream.serializeHeaders() method

[panchenko]

No description provided.

[ejona86]

One of the recent servlet changes has made the Windows CI heavily flaky. I had retried the Windows CI 4-ish times and it failed for all of them.

It's producing an INTERNAL error. I've sent out #12317 to get a better view of what error is occurring.

[ejona86]

#12317 humorously succeeded the first time. After re-running to get the CI to fail, I was able to see the error for tomcat10Test's TomcatInteropTest.timeoutOnSleepingServer:

Status{code=INTERNAL, description=Received unexpected EOS on empty DATA frame from server, cause=null}