#206 Fix splitting path fragments to return a mutable list

[adisandro]

This fixes the first issue described in #206.

[abelhegedus]

Since the code is under EPL, can you state the following before we merge this:

• Did you author 100% of the content?
• Do you have the rights to contribute this content to Massif?
• Is the contribution under the Eclipse Public License?

I know it's a single line, but it's best to be on the safe side.

[adisandro]

Yes to all of the questions!

[abelhegedus]

Thank you for your contribution!