Don't wrap request bodies if they are already Readables

We don't need to wrap request bodies if they already conform to the `::Protocol::HTTP::Body::Readable` interface. This way developers can use other kinds of `Readable`s directly when making calls with `Faraday`.

lib/async/http/faraday/adapter.rb

@@ -118,7 +118,9 @@ def call(env)
 						if body = env.body
 							# We need to wrap the body in a Readable object so that it can be read in chunks:
 							# Faraday's body only responds to `#read`.
-							if body.respond_to?(:read)
+							if body.is_a?(::Protocol::HTTP::Body::Readable)
+								# Good to go
+							elsif body.respond_to?(:read)
 								body = BodyReadWrapper.new(body)
 							else
 								body = ::Protocol::HTTP::Body::Buffered.wrap(body)