Investigate whether Collectors2AdditionalTest.sumBy*Parallel is thread-safe.

[motlin]

I saw this one-off error in Travis and I don't want to lose track of it.

13:38:40:796 [ERROR] Tests run: 64, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 1.594 s <<< FAILURE! - in org.eclipse.collections.impl.collector.Collectors2AdditionalTest
13:38:40:796 [ERROR] sumByDoubleParallel(org.eclipse.collections.impl.collector.Collectors2AdditionalTest)  Time elapsed: 0.036 s  <<< ERROR!
java.lang.ArrayIndexOutOfBoundsException
	at org.eclipse.collections.impl.collector.Collectors2AdditionalTest.sumByDoubleParallel(Collectors2AdditionalTest.java:225)
Caused by: java.lang.ArrayIndexOutOfBoundsException: 1

13:39:23:932 [ERROR] Errors: 
13:39:23:933 [ERROR]   Collectors2AdditionalTest.sumByDoubleParallel:225 » ArrayIndexOutOfBounds
13:39:23:933 [INFO] 
13:39:23:933 [ERROR] Tests run: 149942, Failures: 0, Errors: 1, Skipped: 0

[nikhilnanivadekar]

I had noticed this one before as well. I disqualified it as one off but I guess it's persistent. I was not able to replicate it in local to actually fix it