Fix: File target doesn't duplicate header in archived files #245
Add test and fix(2) for #245

Fix for method FileTarget.MoveFileToArchive where a check for an
existing fileName in the dictionary was only for the filename and not
for the existing keys that contains the full path.
Added a Unit Test
Add NLogTestBase.AssertFileContentsStartsWith(..)

Add NLogTestBase.AssertFileContentsStartsWith(..) for checking the begin
of the logfile (for instance the header).
Merge pull request #338 from cvanbergen/master

Fix: File target doesn't duplicate header in archived files #245