Django client: Fields in buttons are missing from POST data

I have this form:

```

Submit
(some other fields etc...)

```

When I do this:

`browser.find_by_name('foo').first.click()`

On the server I can see that the submitted POST data is missing the field "foo".

I'm using splinter[django]==0.11.0 with Python 3.7 but I recall this happening about a year ago also.

Reproduce the issue using the HTML form and browser.find_by_name('foo').first.click() described in the report, then inspect how the Django client serializes submitted button fields. The work is done when the button's name and value appear in the server's POST data and the behavior is covered by a regression test.

Written by the indexing model from the issue text.