Fixed empty HTTP_REFERER calls.

    When HTTP_REFERER is not set (which is a quite likely scenario), dancer
    would die with:
        isa check for "referer" failed: undef is not a string!
Revert "Fixed empty HTTP_REFERER calls."

This reverts commit ad00b6d1fc2093da9a7b2ec6e80b84d180417fe8.