Removed AWS4AuthRequest Layer | Deprecate HTTP.Post without headers

[mattBrzezinski]

The AWS4AuthLayer was introduced into this package because it was required for signing AWS signatures in HTTP. With #443 we are now able to insert custom layers into the HTTP stack, this means we can now move the AWS4AuthLayer out of this package.

I have already moved the AWS4AuthLayer into AWSCore.jl in this commit. Going forward if you want to use the AWS4Auth Layer in your stack would be to do something like:

stack = stack()
insert(stack(), MessageLayer, AWS4AuthLayer)

request(...)

---

As per #473 this CR will also deprecate the usage of HTTP.Post without headers for body::Form.

[omus]

Changes look good to me. Just noticed a spelling mistake which isn't your fault

[omus]

Appears to be additional changes to make to appease the tests

[mattBrzezinski]

Forgot that we needed to deprecate HTTP.post() without headers in the next minor version. Added the changes to it in

1250ad9

[fredrikekre]

Before releasing a new breaking version it would be good to consider #469.

[mattBrzezinski]

That's a good idea, might as well get in what we can with this release.

[quinnj]

This PR has been noted as desirable before we do a 1.0 release; @mattBrzezinski, any chance you could update it? Even if just the src code and I can help with any test-related changes?

[mattBrzezinski]

This PR has been noted as desirable before we do a 1.0 release; @mattBrzezinski, any chance you could update it? Even if just the src code and I can help with any test-related changes?

Yes. I can have this updated probably by the end of the year some time? Just wrapping up other stuff currently.

[quinnj]

The removal of AWSLayer is now included in #789; I'm not familiar with the post deprecation that was included in this PR, so we might want to do that in a new commit if it's still relevant.