Prior to this commit, the PrintingResultHandler used by the various
print() and log() methods in Spring MVC Test printed the response body
but not the request body.
Since request bodies are sometimes generated programmatically, however,
it can be beneficial to have the dynamically generated request body
logged as well.
This commit therefore prints the request body in PrintingResultHandler
by delegating to the recently introduced getContentAsString() method in
MockHttpServletRequest.
Issue: SPR-14717
487bc7505b