Removed superfluous removing of extension.

This causes problems since there is not supposed to be an extension anyway. Removing an extension from DTCoreText-1.2 causes the package name to be become DTCoreText-1.xar which is wrong.

Fixes #338
Add the .xar extension to the enclosure URL if necessary
Fixes #338

The extension xar is added to the output file path for the docset utility, but it is not checked that it is in the url. Without this there is a scenario where the URL is missing the extension.
Merge pull request #339 from Cocoanetics/atomurlfix

Fixes a bug causing the xar-file URL to be incorrect