Fix for post-ing form data.

[rslsystems]

When using the https://api.cloudflare.com/#dns-records-for-a-zone-import-dns-records endpoint I am getting;
Failed to parse form data. Make sure the Content-Type is multipart/form-data.

It appear to be the force key fo json being applied to all non-get requests is causing the issue.

Issue: https://github.com/cloudflare/cloudflare-php/issues/226