bootstrap3: prepend generic class to input forms to user supplied ones

This let the user passing custom classes to widgets override the
bootstrap defaults.

Author: xrmx

bootstrap3/renderers.py

@@ -203,7 +203,7 @@ def add_class_attrs(self):
                                         RadioSelect,
                                         CheckboxSelectMultiple,
                                         FileInput)):
-            classes = add_css_class(classes, 'form-control')
+            classes = add_css_class(classes, 'form-control', prepend=True)
             # For these widget types, add the size class here
             classes = add_css_class(classes, self.get_size_class())
         self.widget.attrs['class'] = classes