Ensure fallback to global fetch has correct this binding

[eyelidlessness]

See #281. As I mentioned in review there:

• I expect this also affects fetchFormAttachment. This is less urgent, because that fallback isn't particularly useful unless/until we add offline support (where a fetch call handled by a Service Worker may very well be how we handle form attachments).
• We should add tests covering these cases, and anywhere we indirectly reference the global fetch. (Bonus points if we can find a way to automate preventing the same mistake again, as it's a pretty easy one to make.)