Refactor treatment of request-method (API Change):

fix #397

method enum class is added to represent all known request methods.
Functions are provided to convert from strings to and from the method
enumeration.

The fields container is modified to also work with the enum.

test/http/message.cpp

@@ -199,7 +199,7 @@ public:
     {
         {
             request<string_body> m;
-            m.method("GET");
+            m.method(verb::get);
             m.target("/");
             m.version = 11;
             m.fields.insert("Upgrade", "test");