Fix default test names for headers and redirection test methods

The default test names in `response_headers_include` and `response_redirect_location_is` assume `$req` is an array reference, so these methods fail when a `Dancer::Response` is passed and no test name is provided:
```perl
my $resp = dancer_response GET => '/someurl';
response_headers_include $resp, [ someheader => 'somevalue' ]; # Not an ARRAY reference at ...
```
`$req` should be wrapped in `_req_label` (as it is in other test methods) -- this commit fixes that.