Add content field to WSGIRequest proxy

The replacement WSGIRequest class defines status_code, but does not define other fields. Our code uses request.content extensively, and we're getting a lot off errors like
```
apps/accts/tests/test_api.py:122: [E1103(maybe-no-member), OrganizationTestCase.test_user_stats] Instance of 'WSGIRequest' has no 'content' member (but some types could not be inferred)
```
This pull request adds a content field to the replacement WSGIRequest object.