Handle incorrectly encoded responses from Barclays EPDQ

This PR is to fix #588 

The encode method used there by @aprofeit won't work in Ruby 1.8.  In Ruby 1.8, you need to use iconv.  Then in Ruby 1.9, iconv is deprecated, and you need to use encode.

I've used a conditional statement here that uses iconv for now, then will switch to the encode method in Ruby 1.9.  I'm not sure if that's the ideal solution, but without it this will break again when active merchant is upgraded to 1.9.

Review: @ntalbott @jduff