Prior to this commit, calls to setHeader() and addHeader() in
MockHttpServletResponse would result in an IllegalArgumentException or
NullPointerException if the supplied header value was null.
Although the Javadoc for setHeader(String, String) and
addHeader(String, String) in javax.servlet.http.HttpServletResponse
does not specify how a null header value should be handled, both Tomcat
and Jetty simply ignore a null value. Furthermore,
org.springframework.http.HttpHeaders.add(String, String) declares the
headerValue parameter as @Nullable.
This commit therefore updates MockHttpServletResponse to silently
ignore null header values passed to setHeader() and addHeader().
Closes gh-26488
3b4a3431d4