Patch aws-sdk's aws/core.rb to avoid failed autoloads.

This fixes LOGSTASH-1718.
Add test coverage for LOGSTASH-1718

This new test verifies that aws-sdk can autoload constants correctly.

I verified this test *fails* before the fix is applied and passes after
it is applied.
Merge pull request #923 from jordansissel/LOGSTASH-1718

Fix aws-sdk require problems.
Merge git://github.com/logstash/logstash