Prior to this commit, there was no way to replace the Set-Cookie header
via MockHttpServletResponse. Specifically, an invocation of setHeader()
for the Set-Cookie header resulted in an additional Set-Cookie header
instead of replacing the existing one, which is in violation of the
contract for javax.servlet.http.HttpServletResponse.setHeader(...).
This commit refactors the internals of MockHttpServletResponse to ensure
that an existing Set-Cookie header is overwritten when set via an
invocation of setHeader(). This commit also verifies the expected
behavior for addHeader() and addCookie() with regard to multiple cookies.
Closes gh-23512
8189c90741