* fix: premature close with chunked transfer encoding and for async iterators in Node 12
This PR backports the fix from #1064 to the `2.x.x` branch following
the [comment here](https://github.com/node-fetch/node-fetch/pull/1064#issuecomment-849167400).
I had to add some extra babel config to allow using the `for await..of`
syntax in the tests. The config is only needed for the tests as
this syntax is not used in the implementation.
* chore: fix up tests for node 6+
* chore: codecov dropped support for node < 8 without shipping major
* chore: npm7 strips empty dependencies hash during install
* chore: pin deps to versions that work on node 4
* chore: do not emit close error after aborting a request
* chore: test on node 4-16
* chore: simplify chunked transer encoding bad ending
* chore: avoid calling .destroy as it is not in every node.js release
* chore: listen for response close as socket is reused and shows warnings
* dev package bump
* test invalid header differently as node 12 no longer accepts invalid headers in response
* add node v10 in travis test list as node 12 has been released
* remove `encoding` as a dependency, throw descriptive Error if textConverted() is used without it in env
* remove rollup ext dep resolution since we don't need it
* switch to programmer error, rm unneeded test conditions, bump timeout for slow CI
* more kill `encoding` dep PR changes
keep blank "dependencies" prop in package.json so rollup's external config func doesn't seize
add ext dep checks back to the rollup config
no implicit var
clarify test comment
* [squash] alter travis cfg to test with and without `encoding`, various
fix devDeps
separate `encoding` tests to their own block
* [squash] fixing nits
* [squash] ci: full matrix of form-data versions and encoding existence
and nits
Alex Potsides
David Frank
Timothy Gu
Jared Kantrowitz
Tino Vyatkin
David Frank