MOD: include zero content-length header if present

[vjoke]

In Brief

This change addresses an issue in the SigV4 signing process where an explicit Content-Length: 0 header would be omitted from the set of signed headers.

Description of Problem

The v4 signer is responsible for building a canonical request, which includes a list of signed headers. The current implementation has two relevant behaviors:
It automatically adds the Content-Length header to the signed headers only if the request body's length is greater than zero (length > 0).
It iterates through the request's existing headers but contains a condition to always skip a header named Content-Length, to prevent it from being added twice.
The combination of these two behaviors means that if a request has a zero-length body and an explicit Content-Length: 0 header, the header is never included in the signature. This can lead to signature mismatch errors for AWS services that require Content-Length to be signed, even for empty bodies (e.g., S3 PutObject).

The Fix

This PR modifies the condition that skips existing Content-Length headers. Instead of unconditionally skipping it, the condition is now strings.EqualFold(k, contentLengthHeader) && length > 0.
This ensures that the pre-existing Content-Length header is only skipped if the signer has already handled it (because length > 0). If length is 0, the signer will now correctly process and sign an explicit Content-Length: 0 header found on the request.

How to Test

This can be verified with a unit test that creates a request with a zero-byte body, adds an explicit Content-Length: 0 header, and asserts that content-length is present in the canonical headers string generated by the signer.