Deprecate NamedRouteCollection#helpers.

This method was removed at 210b338db20b1cdd0684f40bd78b52ed16148b99 but it is
used by third party gems to check if a named route was defined. To help on the
upgrade path on 4.2.0 we bring it back and emit a deprecation warning.
Merge pull request #16533 from lucasmazza/lm-named-routes-helper

Deprecate NamedRouteCollection#helpers.