CHEF-5100: moar func tests

This tests the reuse of the content length validation object and
the situation where the content length of a previous download is
retained while a 403 or other HTTP error fails to re-init the
content length to zero.  The result is the 403 is turned into an
inaccurate content length exception, masking the real error.
fix mixing should + expect rspec syntax in same file
add CHEF::Rest#get func tests
Merge pull request #1385 from opscode/lcg/CHEF-5100

CHEF-5100: moar func tests