fixes bug 918157 - plugin_query must be double escaped

@AdrianGaudebert r?

This change looks simple but was very very hard to spot. 

Basically, any time we take a `forms.Charfield()` input and pass it onto models.py to be sent as a URL to the middleware we need to watch out since that needs to be in the of keys we treat with special care. 

Can you think of any more Adrian?