[url_encoding] cast to unsigned char for isalpha

This crashes the CRT on Windows when the character is out of the 7 bit
range

Change-Id: I8d3ee66c38651a24b0e5832d8c99edc1c6752f7c
Reviewed-on: http://review.couchbase.org/28116
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>