hackney: provide content-length header for empty requests

[aeruder]

This works around a subtle change in hackney between 1.xx and 4.x.x - newer hackney does not provide the (optional by spec!) "content-length: 0" field for 0-length requests. While clients are free to not provide the content-length in these situations, amazon does require "content-length: 0" for 0-byte POSTs. Without this change some requests will fail with a 411 (like writing a 0-byte file to s3).

This change adds the "content-length: 0" header for 1.xx and 4.x.x - both are fine if the header was already present.

We could calculate and add for all requests but as hackney 4.x.x is written now that ends up being a bit of a waste of cycles as hackney always calculates the content-length, it just may not add the result to the headers if the header is already present.

[aeruder]

Fix for #238

[aeruder]

I have a small utility locally that "creates a directory" on s3 (which is really just a 0-byte put_object) and prior to this change get:

an error occurred: {:error, {:unexpected_response, %{body: "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<Error><Code>MissingContentLength</Code><Message>You must provide the Content-Length HTTP header.</Message><RequestId>...

afterwards it is fine.

[aeruder]

I'll also note I've checked this change against hackney 1.25 and 4.6.0 and both are fine with it in place (although obviously 1.25 was fine prior to this change as well).