Allow relaxing of content type restrictions

[Quantumplation]

I'm working in a context where the remote json_ld document is not always served with a content type header (or served with text/plain); this causes the ReqwestLoader to throw an error, because the content type is invalid.

It'd be nice to have an option (or maybe one exists, and I couldn't find it in the documentation) to ignore/assume/remap content types in some way.