Raise an UnauthorizedAccessException on FileInfo.Delete with a directory
Raise an UnauthorizedAccessException on FileInfo.Delete with a directory

When FileInfo was imported in https://github.com/mono/mono/commit/bdd0de7ee52d9675dada0d19985b9daf4edab5bc the check in Delete()
got removed that raised an UnauthorizedAccessException when the FileInfo object holds a path to a directory instead of a file.

It happened to pass the MonoTests.System.IO.FileInfoTest.Delete_Directory test on OSX because unlink returns EPERM there
which got turned into the expected UnauthorizedAccessException, but on Linux it returns EISDIR so the test would fail.

Fix is to put the explicit check for directory back.

mono/referencesource