Test-case for #164, ValidationError.__unicode__(): use 'is' operator
instead of '==' for unset-check

Currently test-case FAILS with a json-instances that have a screaming
__eq__() operator.
Fixes #164, ValidationError.__unicode__(): use 'is' operator instead of
'==' for unset-check

Test-case now passes OK.
Merge remote-tracking branch 'ankostis/master'

* ankostis/master:
  Fixes #164, ValidationError.__unicode__(): use 'is' operator instead of '==' for unset-check
  Test-case for #164, ValidationError.__unicode__(): use 'is' operator instead of '==' for unset-check
Formatting.
Move this test case where it belongs.