Fixed RangeWidget. Used custom template instead of format_output.

[felixxm]

Fixed test_widget and test_widget_attributes i.e.:

======================================================================
FAIL: test_widget_attributes (tests.test_widgets.RangeWidgetTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/django-filter/tests/test_widgets.py", line 145, in test_widget_attributes
    <input type="date" name="date_1" />""")
  File "/django-filter/.tox/py34-djangolatest-restframeworklatest/lib/python3.4/site-packages/django/test/testcases.py", line 708, in assertHTMLEqual
    self.fail(self._formatMessage(msg, standardMsg))
AssertionError:
- <input name="date_0" type="date" /><input name="date_1" type="date" />
+ <input name="date_0" type="date" />-<input name="date_1" type="date" />

Since Django>1.10 Widget.format_output method is removed (see commit b52c7300 and doc).

[carltongibson]

@felixxm Awesome. I was wondering about that one. (Will review Monday) 👍

[rpkilby]

format_output can't be removed yet, since it's required for django < 1.11

[felixxm]

You're right. We need to keep both solutions until Django 1.10 support will be dropped.

[rpkilby]

Unfortunately, 1.8 support ends in April 2018, so that's a ways off.

[carltongibson]

Awesome. Great. Thanks.