[#17] Allow manually sending "body" in request init

[danawoodman]

Other changes:

• Add test for changing content-type via request init to verify behavior.
• Change vitest test environment to jsdom. This removes the need for importing isometric-fetch and brings the test env closer to the DOM which seems a safe baseline test env.

Closes #17

[kwhitley]

Merging this is a work-off point for a PR to modify behavior...

[kwhitley]

Gonna add a few bytes, but hopefully only temporarily

[danawoodman]

@kwhitley just curious, whatcha gonna change?

[kwhitley]

• not embedding application/json by default (to allow blobs to override it)
• not stringifying if instanceof FormData || Blob

Couple tests will have to change, but, although a breaking change (yay for pre-release), no one's code should really break

[kwhitley]

Something like this...

[kwhitley]

would have used passthrough on the body: ... line too but was getting a TS error I was too impatient to sort through, lol