api: fix coprs.models.User usage in search
    
    In 5f785d72 we introduced new search API, but it doesn't work now.
    Partial fix in 063530f1, but we have the same traceback:
    
    Traceback (most recent call last):
      File
    "/usr/lib/python2.7/site-packages/flask/app.py", line 1701, in __call__
        return self.wsgi_app(environ,
    start_response)
      File
    "/usr/lib/python2.7/site-packages/flask/app.py", line 1689, in wsgi_app
        response =
    self.make_response(self.handle_exception(e))
      File
    "/usr/lib/python2.7/site-packages/flask/app.py", line 1687, in wsgi_app
        response = self.full_dispatch_request()
      File
    "/usr/lib/python2.7/site-packages/flask/app.py", line 1360, in full_dispatch_request
        rv = self.handle_user_exception(e)
      File
    "/usr/lib/python2.7/site-packages/flask/app.py", line 1358, in full_dispatch_request
        rv = self.dispatch_request()
      File
    "/usr/lib/python2.7/site-packages/flask/app.py", line 1344, in dispatch_request
        return
    self.view_functions[rule.endpoint](**req.view_args)
      File
    "/usr/share/copr/coprs_frontend/coprs/views/api_ns/api_general.py", line 416, in api_coprs_search_by_project
        jsonout = flask.jsonify(output)
      File
    "/usr/lib/python2.7/site-packages/flask/helpers.py", line 198, in jsonify
        indent=None if request.is_xhr else
    2), mimetype='application/json')
      File
    "/usr/lib64/python2.7/json/__init__.py", line 250, in dumps
        sort_keys=sort_keys, **kw).encode(obj)
      File
    "/usr/lib64/python2.7/json/encoder.py", line 209, in encode
        chunks = list(chunks)
      File
    "/usr/lib64/python2.7/json/encoder.py", line 434, in _iterencode
        for chunk in _iterencode_dict(o,
    _current_indent_level):
      File
    "/usr/lib64/python2.7/json/encoder.py", line 408, in _iterencode_dict
        for chunk in chunks:
      File
    "/usr/lib64/python2.7/json/encoder.py", line 332, in _iterencode_list
        for chunk in chunks:
      File
    "/usr/lib64/python2.7/json/encoder.py", line 408, in _iterencode_dict
        for chunk in chunks:
      File
    "/usr/lib64/python2.7/json/encoder.py", line 442, in _iterencode
        o = _default(o)
      File
    "/usr/lib64/python2.7/json/encoder.py", line 184, in default
        raise TypeError(repr(o) + " is not
    JSON serializable")
    TypeError: 
    0x7f7544056a90> is not JSON serializable
    
    Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>