UnmatchedOrder.matched_date is not serialisable

[agberk]

I've run into an issue using the order stream which results in exceptions like the following (order_queue is a Queue which is the output_queue for a StreamListener):

Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 754, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/aaron/workspace/xxx/xxx/order_feed/order_feed.py", line 35, in run
    order_book = self.order_queue.get()
  File "/usr/local/lib/python2.7/dist-packages/betfairlightweight/resources/baseresource.py", line 63, in json
    return json.dumps(self._data)
  File "/usr/lib/python2.7/json/__init__.py", line 244, in dumps
    return _default_encoder.encode(obj)
  File "/usr/lib/python2.7/json/encoder.py", line 207, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python2.7/json/encoder.py", line 270, in iterencode
    return _iterencode(o, 0)
  File "/usr/lib/python2.7/json/encoder.py", line 184, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: datetime.datetime(2017, 2, 16, 11, 18, 59) is not JSON serializable

On further testing, this appeared to happen when the CurrentOrders returned from the OrderBookCache contained an UnmatchedOrder which had a matched_date field that was not None (it was a datetime.datetime object).

As you can see from the placedDate field a few lines down, that datetime.datetime object is converted to its string representation when being serialised:

https://github.com/liampauling/betfairlightweight/blob/master/betfairlightweight/resources/streamingresources.py#L425

The terminology as described in the documentation is quite confusing:

http://docs.developer.betfair.com/docs/display/1smk3cen4v3lu3yomq5qye0ni/Exchange+Stream+API#ExchangeStreamAPI-OCM/OrderChangeMessag

It seems like an UnmatchedOrder can contain a matched_date in the cases of partial matches.

I've created a pull request which contains the change I made locally to get it to work - you may have a nicer way of dealing with it.

#39

[liampauling]

Weird that I have never encountered this, will merge asap.