Prior to this commit, MockHttpServletResponse's setCharacterEncoding()
method did not update the contentType property, which violates the
Servlet 2.4 Javadoc for getContentType() and setCharacterEncoding().
This commit addresses this issue; however, some existing tests may have
to be updated as a result of this change.
For example, note how some of the tests in this commit have been
refactored to use MediaType##isCompatibleWith() instead of asserting
exact matches for the value returned by MockHttpServletResponse's
getContentType() method.
Closes gh-25536
5de549d7d4