Handle DriverWrapper when scanning registered drivers in JDBWrapper.getConnector()

[JoshRosen]

This is a followup to #143 which fixes a corner-case bug in our scanning of registered JDBC drivers: we need to properly handle Spark's DriverWrapper drivers, which are used to wrap JDBC drivers in order to make them accessible from the root classloader so that the DriverManager can find them.

A simpler, reflection-free version of this change was incorporated into apache/spark#10519

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Merging #147 into master will not affect coverage as of 61c04da

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Files 13 13 Stmts 638 638 Branches 140 140 Methods 0 0 ======================================
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[JoshRosen]

Unfortunately, this is a little tricky to test in an automated fashion because we need to rig the classloaders in order to make sure that the test JDBC drivers aren't loaded by the root classloader.

As a pragmatic trade-off, I'm inclined to test this manually using spark-shell and --packages with the Postgres driver. I'll do this tomorrow.

[JoshRosen]

I tested this manually with spark-shell, --jars, and a modified copy of this code with additional logging statements. I ran my manual tests on Spark 1.4.1, 1.5.1, and 2.0.0-SNAPSHOT, so I'm confident that the portions of the reflection code here which aren't covered by unit tests will work in those versions.

In case we don't get automated tests for this in place by the time of the next release, I'll open a PR to document the manual testing procedure and will create a checklist so future release coordinators / maintainers can know how to ensure that this doesn't break.

I'm going to merge this now and will begin preparation for a new spark-redshift release tomorrow.