Add mach.contentCharSet

As discussed in #42.

I copied the implementation of `mach.contentType`.  

I'd like to add it to the default stack, but I couldn't find out where that happens.  (Something is setting "Content-Type" by default when the user hasn't.)
This middleware should check to see if the response already specifies a charset. If it does, leave it alone. If not, use the one given in the arguments to the middleware.

I'm thinking this is probably all we actually need to satisfy your original request, which was to default to unicode.