fix: bug where FormData in the js fetch target wasn't applied

[erunion]

• Fixes a bug in the JS fetch target where FormData was being set to a form variable but never actually inserted. into the body of the fetch call.
• Stops the JS fetch target from setting an empty object of headers.
• Slightly cleans up the noise around the actual fetch call and indents the promise handling.

[CLAassistant]

All committers have signed the CLA.

[jbwyme]

Hey just wanted to check in on the status of this! I believe this is causing incorrect code to be generated in our ReadMe docs.

[erunion]

@jbwyme You've already got this fix if you're a ReadMe customer because we run a fork (http://npm.im/@readme/httpsnippet) that have these changes. What's the bug you're seeing?

[jbwyme]

Ah gotcha, perhaps this is unrelated. Here's the loom I shared with support: https://www.loom.com/share/06da93107f4b40f88172d680507ac99d

[erunion]

@jbwyme Gotcha, that hit my desk yesterday.

[reynolek]

Fixed merge conflict injected from the PR that removed node 8