Make OMD picklable under py3, fixes #337

[mahmoud]

For some reason this test passed under py2, which I guess is telling as to how long it's been since I was actively pickling.

[allanlei]

There seems to be additional pickling issues. The deserialized value loses the multi-value for getlist.

In [48]: value = OrderedMultiDict([('a', 1), ('a', 2), ('b', 3)])

In [49]: value.getlist('a')
Out[49]: [1, 2]


In [50]: pickle.loads(pickle.dumps(value)).getlist('a')
Out[50]: [2]

# `iteritems` seems to be correct
In [16]: list(value.iteritems(multi=True))
Out[16]: [('a', 1), ('a', 2), ('b', 3)]

In [11]: list(pickle.loads(pickle.dumps(value)).iteritems(multi=True))
Out[11]: [('a', 1), ('a', 2), ('b', 3)]

[mahmoud]

Right you are. Odd that Py2 was so much better behaved than Py3.

Just a couple implementation notes:

• The __getnewargs__ approach in the docs almost works, but because OMD inherits from dict, the builtin implementation overwrites the values via __setitem__ calls after __new__. That's why only individual values made it through; OMD's __setitem__ clears previously-added values.
• __setstate__ apparently runs after the __setitem__ calls mentioned above, allowing us to reset the multi-value. This does mean that there's some extra work happening during unpickling. Maybe a fancy __reduce_ex__ could get around this, but the current approach seems fine for now.