503 Must Use Post Return Code for 2012 RDS Gateway Implementation

I have a new Microsoft RDS implementation at my company and am having some troubles getting the RDS Gateway health monitors to work correctly. I am using the two Windows 2012 health monitors detailed in F5's own RDS Deployment Guide found at [http://www.f5.com/pdf/deployment-guides/f5-microsoft-remote-desktop-services-dg.pdf]. The RDS Gateway health monitor steps are located on page 10 and 11.

When I test these health monitors via the CLI, I am receiving a "HTTP/1.0 503 Must use POST" return. I also placed the same receive string "HTTP/1.0 503 Must use POST" into the Health monitors on the F5 just to make sure the F5 wasn't seeing something different than my CLI testing, attached it to the pool, and the pool came right up fine. When testing from the CLI, I am using the following command (less some company details). For testing purposes, I have made rdshealthmon a domain admin so permissions are not an issue.

curl -k https://172.16.3.140/Rpc/ -H "User-Agent: MS-RDGateway/1.0\r\nRDG-ConnectionId: {00000000-0000-0000-0000-000000000000}\r\nHost: rdsgate.companyname.com\r\nConnection: Close\r\n" --basic -u "company\rdshealthmon" -v

I am wondering if anyone else has had this issue while following F5's own Deployment guide and if there is a solution around it.