Not sure this is the best way to go about this.
This will throw an error if you pass an http method other than what's in HttpMethods.
The HTTP standard only has 1 required method and that's the OPTIONS method. The rest of them are essentially just common conventions.
You can see Apache's Java HttpClient uses a string for that parameter as well instead of an Enumerable.
Here's a discussion that comes up a lot as to why:
http://stackoverflow.com/questions/6722248/why-httprequest-httpmethod-is-string-instead-of-enum