fix: Omit the Content-Type header for requests without bodies

[ashovlin]

Description

Currently if the generated marshaller doesn't set the Content-Type header for a given request AND the request uses an HTTP verb that could have a body, we set a Content-Type header here:

aws-sdk-net/sdk/src/Core/Amazon.Runtime/Pipeline/Handlers/Marshaller.cs

Lines 89 to 98 in 1baa759

	if (method != "GET" && method != "DELETE" && method != "HEAD")
	{
	if (!requestContext.Request.Headers.ContainsKey(HeaderKeys.ContentTypeHeader))
	{
	if (requestContext.Request.UseQueryString)
	requestContext.Request.Headers[HeaderKeys.ContentTypeHeader] = "application/x-amz-json-1.0";
	else
	requestContext.Request.Headers[HeaderKeys.ContentTypeHeader] = AWSSDKUtils.UrlEncodedContent;
	}
	}

This is problematic for at least one service (Connect Outbound Campaigns) that throws 415: Unsupported Media Type when it receives a Content-Type for a POST request with no body.

Now, instead of setting Content-Type based on the verb, we set it based on if the request has a body. I think this is what we intended to do in e6f0f5e, but missed that we have this fallback logic in the core marshaller.

Motivation and Context

V1399441092

Testing

Added a new test case to our mock rest-json service to test that a POST without a body doesn't have a Content-Type header

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license