restore missing assertion in remote_file http spec

Cleans up the test to use let bindings everywhere. Motivation for
changing this file is to fix CHEF-3140 regression test. Test needs to
assert that Chef::REST is getting created with the disable_gzip option.
Because of overlapping method expectations, this assertion was commented
out. To make the tests more resilient to this kind of mistake, expected
arguments were added to the method call expectation for
Chef::REST.new().
fix warning for space between method and parens

No behavior changes. Fixes ruby warning "() interpreted as grouped
expression" that results from leaving a space between method call and
parens, e.g., `call_a_method (with, these, args)`.
remove redundant tests from remote_file provider

I have gone through these tests one-by-one to confirm that the behavior
they intended to test is still tested elsewhere. See content_spec.rb,
fetcher_spec.rb, and http_spec.rb under spec/unit/provider/remote_file.