Cache regexp source on format validation to avoid allocating new objects

Example:

    >> r = /some-regexp/
    => /some-regexp/
    >> r.source.object_id == r.source.object_id
    => false

activemodel/lib/active_model/validations/format.rb

@@ -33,8 +33,8 @@ def record_error(record, attribute, name, value)
       end
 
       def regexp_using_multiline_anchors?(regexp)
-        regexp.source.start_with?("^") ||
-          (regexp.source.end_with?("$") && !regexp.source.end_with?("\\$"))
+        source = regexp.source
+        source.start_with?("^") || (source.end_with?("$") && !source.end_with?("\\$"))
       end
 
       def check_options_validity(options, name)