Allow symbol matchers (to make README examples actually work)

The :method examples stated in the README don't work because no matcher for symbols exists:

    # redirect GET's one way
    r301 "/players", "/current_players", :method => :get

This rule never matches because the method in rack_env is "GET", thus :get is not equal. This little patch accepts symbol matchers, converts both legs to string and downcases them. Symbol matchers therefore are case insensitive.