`strip_spaces` fails on `nil` since 1.15.0

[cpiemontese]

Hello!

Since v1.15.0 strip_spaces, and thus Plug.Conn.Utils.content_type, on a nil will fail with an ArgumentError whereas before it simply returned an :error.

I don't know if this is really an issue, given that content_type should be expected to just handle binary, but since I don't see it explicitly documented anywhere I wanted to report it

[josevalim]

I will add a guard but it will continue failing. :) Thank you!