Sungularize ActionController::UnpermittedParameters error in case when only 1 parameter is unpermitted.

Old behaviour of Strong params had grammar error in case single attribute was unpermitted:

`ActionController::UnpermittedParameters (found unpermitted parameters: cellphone)`

This pull-request fixed it, so new error will have singular form:

`ActionController::UnpermittedParameters (found unpermitted parameter: cellphone)`

Of course, if there are 2 or more unpermitted parameter then error will be pluralized:

`ActionController::UnpermittedParameters (found unpermitted parameters: cellphone, town)`