middleware can't have a trailing s
remove compatibility code for Ruby <1.9.2
we won't want duplicated fields in the client! see:

consider this:

class Middleware
  def self.members; [:value]; end
  include RC::Middleware
end

Client = RC::Builder.client(:value) do
  use Middleware
end

Client.value # which one? before this commit, this would have issues.
README.md: add a section for How We Pick the Default Value