Check input parameters formats

[laurentsenta]

Something like that should not happen:

In [12]: J.items.url
Out[12]: "http://storage.scrapinghub.com/items/{u'key': u'xxxx', u'close_reason': u'finished', u'logs': 143, u'ts': 1460461907499, u'finished_time': 1460461908788, u'items': 54989, u'spider': u'jython_ml', u'elapsed': 3246802, u'state': u'finished', u'pending_time': 1460454433352, u'version': u'1460454410', u'pages': 11500, u'running_time': 1460454434626}"