Fix sqlmap accidental codepoint

See http://www.ruby-doc.org/core-1.9.3/String.html#method-i-3C-3C

Apparently, String#<< uses Integer#chr, not Integer#to_s. News to me.

Fixed originally by @TsCl in PR #2435, but fixing seperately in order to
avoid screwing up his downstream tracking. Note, this isn't a merge, so
using Closes tag on the commit message.

[Closes #2435]