Issue 1754: Chef::Exceptions::Env exception when modifying env resource ...

...using key_name attribute that differs in case from the environment variable name (https://github.com/opscode/chef/pull/1754).

@sersut, @btm, @danielsdeleo, @jdmundrawala, the issue here is that env var names are case insensitive on Windows, and we correctly handled this in earlier versions of Chef prior to the wmi-lite changeover. Most usages were drop-in replacements, but I made a change to use `instances_of` rather than perform a WQL query as in the earlier code (https://github.com/opscode/chef/blob/11.12.8/lib/chef/provider/env/windows.rb#L55,L56).

This change goes back to the original behavior of doing a query, so that comparison is handled by WMI . It also eliminates the need for code that enumerates each environment variable in search of the right one.

See comments in the added functional tests that highlight the example that fails without this fix.
I'm not testing for that actually -- I had started to do that but removed that from the spec since it has never been any sort of explicit or implied requirement. I'll change the description.