Allow report to filter for a key multiple times
    
    When we had an url with a query such as:
      ?milestone=none&milestone=1.0.1
    We were saving it as:
      {'milestone': '1.0.1'}
    therefore loosing half of the information and that due to the code
    assuming one value per key.
    
    With this commit the url query:
      milestone=none&milestone=1.0.1
    is saved as:
      {'milestone': ['none', '1.0.1']}
    as desired.
    
    Fixes https://pagure.io/pagure/issue/2012