OverflowError with waveform PVs

[mattgibbs]

Reported by Mike Lafky: OverflowError is thrown when trying to monitor some waveform PVs.

OverflowError: Maximum recursion level reached
Traceback (most recent call last):
File "_ctypes/callbacks.c", line 315, in 'calling callback function'
File "/usr/lib/python2.7/epics/ca.py", line 490, in _onMonitorEvent
args.usr(value=value, **kwds)
File "/usr/lib/python2.7/epics/pv.py", line 428, in __on_changes
self.run_callbacks()
File "/usr/lib/python2.7/epics/pv.py", line 438, in run_callbacks
self.run_callback(index)
File "/usr/lib/python2.7/epics/pv.py", line 457, in run_callback
fcn(**kwd)
File "./server.py", line 84, in monitor_update_callback
subscriber.ws.send(ujson.dumps(response))

This appears to be an issue with ujson encoding numpy array types.