assertHTMLEqual gives a confusing error message with empty attributes

[Baptiste Mispelon]

If you try the following assertion:
`
self.assertHTMLEqual('<input value>', '<input value="">')
`

You get a test failure and the following message:
`
AssertionError: <input value> != <input value>

<input value>

`

I'm getting mixed signals here: either the test should pass or the error message should show the difference between the two strings.

I'm not actually sure which option would be correct in this case so I'm leaving the ticket as "uncategorized" instead of "bug" or "cleanup".