Small differences with http.FileServer, should be in the readme?

[bezineb5]

Thanks for the excellent library! Two small things worth signalling which make it not a perfect drop-in:

• In case the mime type cannot be determined from the extension by the mime.TypeByExtension method, ServeContent will try to infer from the actual file content: https://golang.org/src/net/http/fs.go?s=4932:5036#L198 so it will return a type like "application/x-gzip", whereas http.FileServer would have return "text/plain" for a YAML file, for example. This can lead to bugs, for example when trying to use the Swagger UI. => workaround: register such non-standard types (there's actually no official mime type for YAML): mime.AddExtensionType(".yaml", "application/x-yaml")
• http.FileServer can find a file requested as "static/test.html" (for example when using http.StripPrefix) whereas gzipped.FileServer requires the leading "/"

Thanks! Ben

[bezineb5]

My bad for the 2nd difference, the issue is with the withIndexHTML helper, in the case "" is requested. I'll do a quick PR.