[HttpFoundation] added class constants in Request for HTTP request methods

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR        | none

Also added constants for methods OPTIONS, TRACE and CONNECT according to [rfc2616](http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html).
I don't realy see the need for using the constants here. The strings are more readable IMO for this case, by having only the method name itself