Added JsonSerializerSettings to refitSettings

Refits will now use a jsonSerializerSettings object to control how to
serialize objects.

This allows the refit settings to be more finely tuned without abusing global state. That way if I want to set that my Refit for this api uses CamelCase, but for another api uses SnakeCase I don't have to worry about them colliding on each other. 
I think it's okay to be lazy about it - I don't think it's a problem having `RefitSettings = settings ?? new RefitSettings();`