Ensure IPN can be verified under Ruby 2.0

Fixes the issue mentioned at https://github.com/tc/paypal_adaptive/issues/75#issuecomment-31033418 by forcing header 'Accept-Encoding: identity'. This has been tested under Rubies 1.9.2, 1.9.3, 2.0.0.

[This suggestion](https://github.com/sdsykes/fastimage/pull/14#issuecomment-30126246) did not appear to work.

References:
http://globaldev.co.uk/2013/03/ruby-2-0-0-in-detail/#_performance_improvements
https://github.com/speedmax/fastimage/commit/181254e72e098c42d4fd7c79ee45612079f36f1b
http://www.ruby-doc.org/stdlib-2.0.0/libdoc/net/http/rdoc/Net/HTTP.html#class-Net::HTTP-label-Compression
http://www.ruby-doc.org/stdlib-2.0.0/libdoc/net/http/rdoc/Net/HTTP.html#class-Net::HTTP-label-Setting+Headers