Panic on unsupported HTTP method

[yurivish]

Right now it's possible for routes to be added to methods that will never be called. I happened to do this since the readme implicitly encouraged defining methods as strings, and I accidentally made a typo and saw that it wasn’t caught at runtime. This 3-line PR will identify the error and panic at route definition.

Thanks for writing this package, by the way – I'm learning how to develop web services in Go using your books and am considering using this router in a production setting since I expect my service won't be particularly CPU-bound and I enjoy the minimal-dependency life. :-)

Edit: Hmm, maybe the error message should say "invalid HTTP method" rather than "unsupported".

Edit 2: An alternative might be to change the example code to use http.MethodGet, which would make it impossible for this sort of typo to compile.

[yurivish]

Actually, I'm not even sure this is a bug – the "typo" methods are indeed accessible via HTTP, and maybe there are niche uses cases for such functionality.