Some providers freeze their name (e.g.
https://github.com/zquestz/omniauth-google-oauth2/blob/414c43ef3ffec37d473321f262e80f1e46dda89f/lib/omniauth/strategies/google_oauth2.rb#L1),
so trying to modify the string would fail with a `can't modify frozen
String` exception (see
https://gitlab.com/gitlab-org/gitlab-ce/issues/37845#note_40308148).

In this case, we can just stop trying to force the encoding of the
provider name as they should always be in utf8 by default.
Signed-off-by: Rémy Coutable <remy@rymai.me>