Concurrency issue when setting custom variables

[d4rken]

Seems to be very rare, happened only a few times out of a few 10k events.
Writing this down so I don't forget it :).
Maybe fix this and do #69 at the same time?

java.util.ConcurrentModificationException
        at java.util.HashMap$HashIterator.nextEntry(HashMap.java:806)
        at java.util.HashMap$EntryIterator.next(HashMap.java:843)
        at java.util.HashMap$EntryIterator.next(HashMap.java:841)
        at org.json.JSONObject.<init>(JSONObject.java:128)
        at org.piwik.sdk.CustomVariables.toString(CustomVariables.java:90)
        at org.a.a.e.a(Tracker.java:16684)
        at eu.thedarken.sdm.tools.e.a.a(PiwikHelper.java:13159)
        at eu.thedarken.sdm.tools.piwik.PiwikHelper$1.run(PiwikHelper.java:305)
        at java.lang.Thread.run(Thread.java:841)

[dotsbb]

I would like to take care of this one, if You don't mind

[d4rken]

Sure 👍, happy hacking.

[d4rken]

Fixed by @dotsbb in #90