ServletServerHttpRequest now falls back on the contentType and the
the characterEncoding of the ServletRequest, if the headers of the
incoming request don't specify one. Similary ServletServerHttpResponse
sets the contentType and the characterEncoding of the ServletResponse
if not already set.
This allows using the CharacterEncodingFilter to set a character
encoding where the request doesn't specify one and have it be used
in HttpMessageConverter's.
SPR-9096
edc80ffa95