Fix win32 buildbreak

Using isalpha results in link problems. Use isupper/islower
instead.

Change-Id: I05bf241bdfca0e641490f2d376547d9af4aa6101
Reviewed-on: http://review.couchbase.org/34489
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>