int & files in a form

The integer on a field don't seem to work when a file is uploaded. I was using django-webtest but the crash occurs in webtest and it seems to be the source of that small bug:

copied from here: https://github.com/kmike/django-webtest/issues/18

I used to have a form like this with the test passing:
form = response.form
form['structure-total_number_of_employees'] = 50
...
response = form.submit()

Then when I've added a file to that and updated my code:
form = response.form
form['logo'] = ...
form['structure-total_number_of_employees'] = 50
...
response = form.submit()

It crashed with the following error:
virtual_env/dev_env/lib/python2.7/site-packages/webtest/app.py", line 382, in encode_multipart
body = b'\r\n'.join(lines)
TypeError: sequence item 12: expected string, int found

To get it to work, I've replaced 50 by "50":
form = response.form
form['logo'] = ...
form['structure-total_number_of_employees'] = "50"
...
response = form.submit()

I reckon that the encode multipart should be updated to handle integers.

Thanks !

Start in webtest/app.py at encode_multipart and reproduce the form submission described with an uploaded file and an integer field. Confirm that multipart encoding handles the integer without the reported TypeError, and add or update coverage for this case if the repository has a relevant test location.