Added two tests related to #174 - proxy generator throws exception when a source property is null.
Changed ProxyGenerator to properly create proxy when a source property is null.  Added appropriate tests.
Created private class to use instead of Tuple in ProxyGenerator.  Added AAA comments to tests.
Renamed class data class per feedback.
Merge branch '174-proxy-generator-null-source' of https://github.com/dlongest/AutoFixture into dlongest-174-proxy-generator-null-source