Add a QueryDict.fromkeys() method

[wim glenn]

In django.http.QueryDict, we currently have the following weird behaviour and misleading error messages:

>>> QueryDict.fromkeys(['k1', 'k2'])
AttributeError: This QueryDict instance is immutable

It is not possible to pass the mutable kwarg to the initialise in this way, either:

>>> QueryDict.fromkeys(['k1', 'k2'], mutable=True)
TypeError: fromkeys() takes no keyword arguments

Django's querydict is-a dict, so the classmethod factory "fromkeys" should either
a) work, or
b) be explicitly disabled in the subclass, and documented as such

The relevant section in the docs ( ​https://docs.djangoproject.com/en/dev/ref/request-response/#django.http.QueryDict ) says that:

QueryDict implements all the standard dictionary methods because it’s a subclass of dictionary. Exceptions are outlined here: ...

However, there is no mention about the fromkeys method.

[wim glenn]

​https://github.com/django/django/pull/6715

[Philip James]

Noted in the PR that I think the current implementation needs more error handling, so I'm marking 'patch needs improvement'. Totally willing to be corrected on this!

[Tim Graham]

Looks good, pending a few cosmetic updates noted on the PR.

[Tim Graham <timograham@…>]

In 5ebebd11:

Fixed #26707 -- Added QueryDict.fromkeys()