Fix rack env spec: check rack.errors instead of rack.input

rack-test 2.x does not populate rack.input in the Rack environment for
requests that have no body (e.g. GET). rack.input is only added when there
is a request body to expose. The test was asserting that the adapter
correctly passes the full Rack env through to the resource; rack.errors is
always present regardless of HTTP method or Rack version, so use that as
the representative Rack-specific key instead.

Author: orien

spec/webmachine/adapters/rack_spec.rb

@@ -57,7 +57,9 @@
 
     it 'provides the rack env on the request' do
       rack_response = get 'test', nil, {'HTTP_ACCEPT' => 'test/response.rack_env'}
-      expect(JSON.parse(rack_response.body).keys).to include 'rack.input'
+      # rack-test does not populate rack.input for GET requests in Rack 3,
+      # so check for rack.errors which is always present in every Rack version.
+      expect(JSON.parse(rack_response.body).keys).to include 'rack.errors'
     end
   end
 end