Fix ExAws.Request.Req: don't rewrite bodyless GETs to POST

[gernotkogler]

Closes #1246.

Req 0.7 automatically converts a GET into a POST when a non-nil request body is set. ExAws calls its HTTP client with "" for bodyless requests (every S3 GET is built with body: data[:body] || ""), and "" != nil, so every GET made through ExAws.Request.Req goes out as a POST. Because SigV4 is signed over the original GET, the server rejects it with SignatureDoesNotMatch (or, for non-signed backends, 405).

In Req's model "no body" is nil, not "", so the adapter just needs to translate ExAws's convention:

body = if body in [nil, ""], do: nil, else: body

Changes:

• fix the shipped ExAws.Request.Req adapter (normalise "" → nil)
• add a regression test: a bodyless GET must reach the server as a GET (verified failing as POST under req 0.7.2 without the fix, passing with it)
• apply the same fix to the Req adapter example in the ExAws.Request.HttpClient moduledoc

Only :get is affected by Req's rewrite, so HEAD/PUT/POST/DELETE are unchanged.

[sjmadsen]

This kind of addresses #1246, but it would be even better if it fixed the existing Req adapter already in ExAws:

ex_aws/lib/ex_aws/request/req.ex

Line 19 in 48785c6

def request(method, url, body \\ "", headers \\ [], http_opts \\ []) do

[gernotkogler]

Good call — I hadn't spotted the shipped ExAws.Request.Req module (it postdates the version I was on). I've now fixed it there as the primary change, added a regression test (verified it fails as POST under req 0.7.2 without the fix), and kept the matching fix to the moduledoc example. Updated the PR to close #1246.