Fix MockHttpServletResponse HTTP status update

Prior to this commit, one could call the setStatus method on
this Mock object and update the response's status,
even though the sendError method had already been called.

According to the HttpServletResponse Javadoc, sendError() methods
commit the response; so the response can't be written after that.

This commit fixes MockHttpServletResponse's behavior; setStatus
methods do not update the status once the response has been
committed.

Issue: SPR-10414
Add doesNotExist match to HeaderResultMatchers

Prior to this commit, one could not test for the absence of
a specific HTTP header in a response.
Using header("X-Custom-Header", Matchers.nullValue()) would not
work because it tests for an empty value of an existing header.

This commit adds a doesNotExist method on the
HeaderResultMatcher.

Issue: SPR-10771