MultiValueDict behaviour does not match dict behaviour for single values

[Piet van Agtmaal]

Description:
When using a MultiValueDict, retrieving the value for a key with only a single value yields unexpected and inconsistent results compared to a regular dictionary. See the code sample below.

>>> from django.utils.datastructures import MultiValueDict
>>> mvd = MultiValueDict()
>>> mvd['test'] = 1
>>> mvd['test']
1
>>> dict(mvd)['test']
[1]

Expected:
dict(mvd)['test'] yields 1, like mvd['test'] does

Actual:
dict(mvd)['test'] yields [1] instead

[Tim Graham]

I believe this is a duplicate of #12375. Even if the behavior could be changed (I'm not sure what the implementation would look like), backwards compatibility is probably more important. The resolution of the other ticket was to add the MultiValueDict.dict() method in 97f22f29699476d5bcbbf599a3dba22909f4ec85.