Use an enum "HttpMethod" not a string
as this has only a few valid values - "Get", "Post"
and possibly other  http verbs that we don't actually need to handle yet
Also this makes it easier to merge with the .Net 4.5 branch, where we use System.Net.Http.HttpMethod
Enumerating the http methods get, post, put, delete
and tests that they are accepted
Merge pull request #118 from AnthonySteele/httpmethod_enum

Use an enum "HttpMethod" not a string