Previously, JarURLConnection assumed that that URL with which it was
created would contain the absolute path of the underlying jar file.
This meant that when it was created with a relative URL, it could fail
to find an entry or throw a StringIndexOutOfBoundsException.
This commit updates the logic for normalizing the input URL so that
both absolute and relative URLs are supported.
Closes gh-6109
02dbd6e4a7