Add support for new Content-Type

[jozo]

New Content-Type should be "application/csp-report"
https://w3c.github.io/webappsec-csp/

This should be merged (or fixed otherwise) ASAP because current content_type check breaks CSP reporting from new browsers.

[jozo]

Check this @Gee19

[Gee19]

@jozo Thanks for the contribution. Do you mind modifying your code slightly for readability sake?

acceptable_content_types = ['application/json', 'application/csp-report']
if content_type not in acceptable_content_types:

[jozo]

Good point. I also moved it to import level so we won't create the list over and over for every request.