Make xor_string safe for ruby 2.0
Make xor_string safe for ruby 2.0

The default encoding for ruby 2.0 is UTF-8, and the shovel operator << will treat
integers as codepoints. It still should have been safe to shovel integers into a
UTF-8 string, but this would result in unicode codepoint 194 being appended to the
beginning of the string.

retailnext/ruby-radius,retailnext/ruby-radius