CHEF-5198: adding func tests for Chef::HTTP clients

Start of func tests for Chef::REST and Chef::HTTP::Simple

- Chef::REST is missing func tests against JSON endpoints using simple
  GET requests
- HEAD and POST requests are also still missing, etc.
DRY up remote_file func specs
wrap methods in a module
forgot to mix into remote_file specs
Merge pull request #1377 from opscode/lcg/CHEF-5198-3-moar-func-tests

CHEF-5198: adding func tests for Chef::HTTP clients