[SPARK-7756] CORE RDDOperationScope fix for IBM Java

IBM Java has an extra method when we do getStackTrace(): this is "getStackTraceImpl", a native method. This causes two tests to fail within "DStreamScopeSuite" when running with IBM Java. Instead of "map" or "filter" being the method names found, "getStackTrace" is returned. This commit addresses such an issue by using dropWhile. Given that our current method is withScope, we look for the next method that isn't ours: we don't care about methods that come before us in the stack trace: e.g. getStackTrace (regardless of how many levels this might go).

IBM:
java.lang.Thread.getStackTraceImpl(Native Method)
java.lang.Thread.getStackTrace(Thread.java:1117)
org.apache.spark.rdd.RDDOperationScope$.withScope(RDDOperationScope.scala:104)

Oracle:
PRINTING STACKTRACE!!!
java.lang.Thread.getStackTrace(Thread.java:1552)
org.apache.spark.rdd.RDDOperationScope$.withScope(RDDOperationScope.scala:106)

I've tested this with Oracle and IBM Java, no side effects for other tests introduced.

Author: Adam Roberts <aroberts@uk.ibm.com>
Author: a-roberts <aroberts@uk.ibm.com>

Closes #6740 from a-roberts/RDDScopeStackCrawlFix and squashes the following commits:

13ce390 [Adam Roberts] Ensure consistency with String equality checking
a4fc0e0 [a-roberts] Update RDDOperationScope.scala