(MAINT) Mergeback 0.7.1 Release

(MAINT) Ensure is_same check works for nil manifest values

Prior to this commit calling initialize for the dsc base provider
would always set the class variables for cached_canonicalized_resource,
cached_query_results, and logon_failures to empty arrays; when
running with 2 or more resources of a different type, this would
cause the provider to reset these values to the empty arrays, thus
wiping out these caches, especially the cache for canonicalized
resources, which is filled *before* the rest of the Puppet run.

This commit updates the initialize method to instead memoize those
caches, only setting them to empty arrays if they do not already
exist. This way, canonicalized resources from multiple types are
not mysteriously removed from the cache.

…-variables

(GH-97) Memoize class variables in initialize

(MAINT) Prep for 0.7.2 release