Modified the implicit cast from DirectoryInfo to DirectoryInfoBase to correctly return null if asked to cast a null value

Signed-off-by: Martin Evans <martindevans@gmail.com>
Merge pull request #36 from srasch/master

MockDirectory.GetCurrentDir() implemented
Merge pull request #35 from martindevans/master

Modified Implicit cast from DirectoryInfo to DirectoryInfoBase to correctly handle null values