Add extra MIME test to verify default 'q' value.

The extra data point in MimeParseTest ensures that if no 'q' value is provided
with a particular MIME type in an Accept header, then a default value of 1.0 is
assumed.
Explain what to do if UTF-8 not enabled for build.

The build requires that the default file encoding is UTF-8, otherwise it fails
fast. This commit adds an explanation of how to fix the problem to the failure
message, i.e. add -Dfile.encoding=UTF-8 to GRADLE_OPTS.