Fix race condition in HeaderWriterFilter when using asynchronous processing

[justin-tay]

Closes gh-15510

This fixes the issue where the headers are still being modified in the main thread by HeaderWriterFilter while the asynchronous processing might have completed and flushed the response. The AtomicBoolean added previously stops the headers being written twice but doesn't fix this issue.

On Tomcat this sometimes produces a header with an empty name which certain clients reject.

In org.apache.tomcat.util.http.MimeHeaders if the following sequence happens

public MessageBytes setValue(String name) {
  ..
  MimeHeaderField mh = createHeader(); // set in the main thread by HeaderWriterFilter
  mh.getName().setString(name); // not yet set before the asynchronous processing commits in the async thread
  ..
}

With org.apache.coyote.http11.Http11OutputBuffer

public void sendHeader(MessageBytes name, MessageBytes value) {
  write(name); // blank as no name set yet
  headerBuffer.put(Constants.COLON).put(Constants.SP);
  write(value); // blank as no value set yet
  headerBuffer.put(Constants.CR).put(Constants.LF);
}

This causes just : to appear as a header and I ended up getting this in my client.

Caused by: org.apache.http.ProtocolException: Invalid header: : 
	at org.apache.http.impl.io.AbstractMessageParser.parseHeaders(AbstractMessageParser.java:230)