Fix bug: TypeError can't convert Fixnum into String

wmap_target_port is retrieved from datastore['RPORT'], and that's a
Fixnum. But wmap_base_url is treating that like a String, so when a
module uses that function, it's doomed.

See:
http://dev.metasploit.com/redmine/issues/7748
Merge remote-tracking branch 'wchen-r7/type_error_dir_scanner' into rapid7