BaseForm._html_output() uses inconsistent formatting for normal row

[A. Lloyd Flanagan]

Steps to reproduce:

1. Create a form with at least one *HiddenInput* form field.
2. Override a method such as as_p() by calling _html_output() with:
   1. A normal_row parameter containing the string "%(field_name)s"
   2. A row_ender parameter which is *not* the last tag in normal_row
3. Attempt to render the form. This will result in a builtins.KeyError: 'field_name' message.

Under these conditions, BaseForm._html_output() attempts to create a new row to hold the hidden input fields. When it does, it uses normal_row % {..}, but the dictionary does not have the field "field_name" in it. This causes a KeyError. On the other hand, if normal_row ends in a row_ender tag, it doesn't create a new row, and the method succeeds.