broken registration_path when SCRIPT_NAME is not empty string

[btalbot]

Rack sets SCRIPT_NAME when a path is mapped to an app. So, a rack app built like below will route
all requests starting with /myapp to MyApp and everything else to OtherApp. For an HTTP request
like /myapp/foo/bar the MyApp will see SCRIPT_NAME='/myapp' and PATH_INFO='/foo/bar'.

Rack::Builder.app do
  map '/myapp' do
    run MyApp
  end
  run OtherApp
end

The Identity#registration_path and Identity#registration_result methods ignore the script_path
and assume it's always empty-string.

If the identity middleware is mapped to /myapp like above then #registration_path will
return as /auth/identity instead of /myapp/auth/identity. This causes any forms to include
a link to the wrong path and any requests for the correct path to fail the #on_registration_path?
check.

[pboling]

Fixed by #120

[pboling]

@btalbot Released in v3.1.0 & v3.1.1