Prior to this commit, PrintingResultHandler always printed the request
or response body regardless of its content type. For binary content,
however, the output was unreadable and therefore useless.
This commit addresses this issue by only printing the request or
response body if it is "printable" (i.e., if its content type is known
to be text-based such as plain text, HTML, XHTML, XML, JSON, etc.). If
the content type is unknown (e.g., unspecified for the HTTP request in
the test), it is assumed that the body is printable.
Issue: SPR-14776
e65a1a4372