MultiWidget's render method does not "mark_safe" the HTML it returns

[David Tulig]

MultiWidget needs to call the "mark_safe" function on the HTML output before returning or it will not render correctly.

To reproduce the bug:

1. Create a form with the field:
   

forms.SplitDateTimeField(widget=forms.SplitDateTimeWidget)

2. Create a template where you explicitly position the form fields, ie. {{ form.start_datetime.label_tag }} {{ form.start_datetime }} . Do not use as_table or any of the other short cuts.

[Chris Beaven]

Thanks dtulig. I was triaging in reverse and came across #6113 first so I'll dupe as that.