Frozen string cannot change encoding

This was shown in specs but surely this will be happening in application
code as well if this method is passes a frozen string.

We were also trying to force_encode a OmniAuth::AuthHash which had the
very confusing behaviour of returning nil when it was sent a method that
it did not define. Fix that by only force_encoding a String.

lib/gitlab/auth/o_auth/auth_hash.rb

@@ -60,8 +60,7 @@ module Gitlab
 
         def get_info(key)
           value = info[key]
-          Gitlab::Utils.force_utf8(value) if value
-          value
+          value.is_a?(String) ? Gitlab::Utils.force_utf8(value) : value
         end
 
         def username_and_email

lib/gitlab/utils.rb

@@ -22,7 +22,7 @@ module Gitlab
     end
 
     def force_utf8(str)
-      str.force_encoding(Encoding::UTF_8)
+      str.dup.force_encoding(Encoding::UTF_8)
     end
 
     def ensure_utf8_size(str, bytes:)

spec/lib/gitlab/auth/o_auth/auth_hash_spec.rb

@@ -40,7 +40,11 @@ describe Gitlab::Auth::OAuth::AuthHash do
       last_name:  last_name_ascii,
       name:       name_ascii,
       nickname:   nickname_ascii,
-      uid:        uid_ascii
+      uid:        uid_ascii,
+      address: {
+        locality: 'some locality',
+        country: 'some country'
+      }
     }
   end
 
@@ -51,6 +55,7 @@ describe Gitlab::Auth::OAuth::AuthHash do
     it { expect(auth_hash.username).to eql nickname_utf8 }
     it { expect(auth_hash.name).to eql name_utf8 }
     it { expect(auth_hash.password).not_to be_empty }
+    it { expect(auth_hash.location).to eq 'some locality, some country' }
   end
 
   context 'email not provided' do