HStore Form Field falsely report has_changed

[Ramez Issac]

Consider this simple Test

from django import forms
from django.contrib.postgres.forms import HStoreField


class HStoreForm(forms.Form):
    f1 = HStoreField()


class TextForm(forms.Form):
    f1 = forms.Textarea()


form_w_hstore = HStoreForm()
assert form_w_hstore.has_changed() is False #Fails

form_w_textarea = TextForm()
assert form_w_textarea.has_changed() is False #Passes

[Andrea Grandi]

Triaged in this example project ​https://github.com/andreagrandi/django-ticket-24844
Running the tests with: python manage.py test
they fails with:

(django18)➜  ticket_24844_hstore git:(master) python manage.py test
Creating test database for alias 'default'...
E
======================================================================
ERROR: hstore.tests (unittest.loader.ModuleImportFailure)
----------------------------------------------------------------------
ImportError: Failed to import test module: hstore.tests
Traceback (most recent call last):
  File "/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/lib/python2.7/unittest/loader.py", line 254, in _find_tests
    module = self._get_module_from_name(name)
  File "/usr/local/Cellar/python/2.7.9/Frameworks/Python.framework/Versions/2.7/lib/python2.7/unittest/loader.py", line 232, in _get_module_from_name
    __import__(name)
  File "/Users/andrea/Projects/Django/ticket-24844-hstore/ticket_24844_hstore/hstore/tests.py", line 16, in <module>
    assert form_w_hstore.has_changed() is False #Fails
AssertionError


----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (errors=1)
Destroying test database for alias 'default'...

[Andrea Grandi]

I try to fix the bug with this PR ​https://github.com/django/django/pull/4705

[Florian Apolloner <florian@…>]

In 43b2d88:

Fixed #24844 -- Corrected has_changed implementation for HStoreField.

[Tim Graham <timograham@…>]

In f4cb350c:

Added release notes for refs #24844.

[Tim Graham <timograham@…>]

In 62c19a21:

Fixed #24844 -- Corrected has_changed implementation for HStoreField.

Backport of 43b2d88a5b9cfb151ccf7ac861f2750e70c0e2c4 from master