Change middleware to support rails app in subfolders

If two applications run on the same domain

first_rails_app -> in http:://any_domain/
second_rails_app -> in http://any_domain/seccond_application

and in the second one a error is occured then "__better_errors" from the second app should be called.

To provide this the regular expression for the better_errors_call-Method should listen on any "__better_errors" string on any page.