Ensure empty body responses are consistently returned as empty strings.

Zero-byte response bodies can be represented as either nil or '' in ruby.
Most ruby HTTP clients use '' (which, conveniently, allows clients
to assume the body is always a string), but Net::HTTP, for whatever
reason, uses nil.

This change makes it so that all faraday adapters consistently return
'' for zero-byte response bodies.

lib/faraday/adapter/net_http.rb

@@ -38,7 +38,7 @@ module Faraday
           raise Error::ConnectionFailed, $!
         end
 
-        save_response(env, http_response.code.to_i, http_response.body) do |response_headers|
+        save_response(env, http_response.code.to_i, http_response.body || '') do |response_headers|
           http_response.each_header do |key, value|
             response_headers[key] = value
           end

test/adapters/integration.rb

@@ -159,6 +159,11 @@ module Adapters
         end
       end
 
+      def test_empty_body_response_represented_as_blank_string
+        response = get('204')
+        assert_equal '', response.body
+      end
+
       def adapter
         raise NotImplementedError.new("Need to override #adapter")
       end

test/live_server.rb

@@ -46,6 +46,10 @@ class LiveServer < Sinatra::Base
     [200, {}, 'ok']
   end
 
+  get '/204' do
+    status 204 # no content
+  end
+
   error do |e|
     "#{e.class}\n#{e.to_s}\n#{e.backtrace.join("\n")}"
   end