Fix flaky test: ensure the json gem is loaded

It seems there are some tests that result in the json gem being loaded.
If the spec/webmachine/adapters/rack_spec.rb:57 test happens to run before
these tests, the json gem will not be loaded and the test fails. This is
reproducible with the following command.

rspec --seed 34885

spec/webmachine/adapters/rack_spec.rb

@@ -3,6 +3,7 @@
 require 'spec_helper'
 require 'webmachine/spec/adapter_lint'
 require 'rack/test'
+require 'json'
 
 describe Webmachine::Adapters::Rack do
   it_should_behave_like :adapter_lint do