Prior to this commit, the Spring MVC Test framework only provided
support for printing debug information about the MvcResult to STDOUT.

This commit introduces support for logging `MvcResult` details at
`DEBUG` level via the Apache Commons Logging API. In addition, this
commit introduces additional `print(..)` variants for printing debug
information to custom output streams and writers.

Specifically, `MockMvcResultHandlers` has been augmented with the
following new static methods:

 - `log()`
 - `print(OutputStream)`
 - `print(Writer)`

Issue: SPR-13171