Ensure http method is always lowercase

[webmat]

Hello! I've noticed that most of my http metrics have method lowercase (like yabeda-rails' metrics), but sometimes I have metrics where method is uppercase. I assume this is a difference per http library.

Here I'm suggesting a trivial change that would ensure it's always lowercase, to match yabeda-rails. Is there appetite for this kind of normalization?

This is a backwards incompatible change with regards to the metrics themselves. I can work to make the behaviour configurable if you want, although I think this would over-complicate things.