Fix mergeJoinSorter use with BLOBS. The createValue() on input wants to
calculate a length, but wasn't getting the correct underlying datatype.

Add test to verify.
merge tjoneslo: Fix mergeJoinSorter use with BLOBS.

Merge pull request #896 from tjoneslo/akiban-server/fix-mergersort-blob
https://github.com/FoundationDB/sql-layer/pull/896