Fix ImmutableMultiDict handling for python 3.7
    
    For some reason, in python 3.7 `dict()`-call over ImmutableMultiDict
    structure from Werkzeug behaves differently. In the previous versions a
    dict of lists is created (thus representing all the values given to the
    specific URL-encoded query parameter), py37 only creates a dict
    containing the first value in string form.
    
    This patch makes sure the ImmutableMultiDict is converted to a
    dict-of-lists in a foolproof fashion.