Crash when rendering inclusion_tag with takes_context with multiple context processors

I have an inclusion tag that looks something like this:

``` python
@register.inclusion_tag("template.html", takes_context=True)
def tag(context, variable):
context.update({'variable': variable})
return context
```

When rendering it I get the following stacktrace deep into floppyforms:
`ValueError: dictionary update sequence element #0 has length 6; 2 is required`
...because the context that gets sent in via takes_context is a list of dictionaries, not a dictionary. I think it's because that each context_processor adds one dictionary and that Django transparently handles that normally.

Here's the full stacktrace: http://dpaste.com/2FWD49D

It does not happen in floppyforms 1.3.0, but happens in 1.4.1. I think it might have something to do with this commit: https://github.com/gregmuellegger/django-floppyforms/commit/d591b12350af4b734cbcedd6c9becd330204268e

A workaround is to apply flatten_contexts to the inclusion tag:

``` python
@register.inclusion_tag("template.html", takes_context=True)
def tag(context, variable):
from floppyforms.compat import flatten_contexts
context = flatten_contexts(context)
context.update({'variable': variable})
return context
```

Sorry that I don't have time to write a proper testcase right now.

Start with the context-handling path implicated by commit d591b12350af4b734cbcedd6c9becd330204268e and inspect floppyforms.compat.flatten_contexts. Reproduce the inclusion_tag case with takes_context=True and multiple context processors, then add regression coverage showing that the tag receives a usable context and no longer raises the reported ValueError.

Written by the indexing model from the issue text.