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: <coprs.models.User object at
0x7f7544056a90> is not JSON serializable

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
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