Decay argument types in call to do_check_format_string

[marack]

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

This change fixes issue #1292 which causes the UDL template based formatter to fail when passed and lvalue to a user defined type despite an appropriate custom formatter being available.

[vitaut]

Thanks for the PR. std::decay is a bit too strong, so I used remove_cvref_t instead and added a regression test in 422e7b9.