Fix TypeError in LintMiddleware

[RonRothman]

This PR fixes a TypeError in werkzueg.contrib.lint.GuardedIterator.close, triggered by a response that, contrary to what the linter expects, has a non-zero content-length.

                if content_length != 0:
                    warn(HTTPWarning('%r responses must have an empty '
>                                    'content length') % status_code)
E                   TypeError: unsupported operand type(s) for %: 'HTTPWarning' and 'int'

[untitaker]

Thanks!