Fix case that user installs JSON::XS.

$e has typeglob such as value of 'psgi.errors'. Typeglob cannot be represented
in JSON and JSON::XS throws exception and the test is failed as default.
So this patch set 'allow_unknown'. If 'allow_unknown' is set, JSON::XS does not
throw exception and represent as null for not-represented values.

If user installs only JSON::PP, this test is passed.
Merge pull request #2 from syohex/fix_test

Fix case that user installs JSON::XS.