Avoid mutation of string in bs3 support

This will break if the string is frozen.  Found by Ruby 3.4's
string literal modification warning.

lib/forme/bs3.rb

@@ -356,7 +356,7 @@ def call(tag)
           case tag.attr[:type].to_sym
           when :checkbox, :radio, :hidden
             # .form-control class causes rendering problems, so remove if found
-            tag.attr[:class].gsub!(/\s*form-control\s*/,'') if tag.attr[:class]
+            tag.attr[:class] = tag.attr[:class].gsub(/\s*form-control\s*/,'') if tag.attr[:class]
             tag.attr[:class] = nil if tag.attr[:class] && tag.attr[:class].empty?
 
           when :file