This commit revisits the behavior of our `MockHttpServletResponse`
implementation with the javadoc clarifications applied in Servlet 6.1.
Prior to this change, adding or setting an HTTP response header with a
`null` name or value would not have the expected behavior:
* a `null` name should have no effect instead of throwing exceptions
* a `null` value when setting a header effectively removes the entry
from the response headers
Also, this commit ensures that `IllegalStateException` are thrown if
`getWriter` is called after a previous `getOutputStream` (and vice
versa).
Closes gh-34467
75329e6d9d