unique fails on dicts

[CJ-Wright]

from streamz import Stream
source = Stream()
s = source.unique().sink(print)
source.emit({'a': 1})

gives an error

~/dev/streamz/streamz/core.py in emit(self, x, asynchronous)
    243                 thread_state.asynchronous = True
    244             try:
--> 245                 result = self._emit(x)
    246                 return gen.convert_yielded(result)
    247             finally:

~/dev/streamz/streamz/core.py in _emit(self, x)
    224         result = []
    225         for downstream in list(self.downstreams):
--> 226             r = downstream.update(x, who=self)
    227             if type(r) is list:
    228                 result.extend(r)

~/dev/streamz/streamz/core.py in update(self, x, who)
    992     def update(self, x, who=None):
    993         y = self.key(x)
--> 994         if y not in self.seen:
    995             self.seen[y] = 1
    996             return self._emit(x)

TypeError: unhashable type: 'dict'

Is this expected behavior?

[CJ-Wright]

closed in #244