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. 
Name this property `JsonSerializerSettings`, just so it's clearer it won't do anything at all for form posts.