ec2plugin: fallback to utf8 if response encoding not set

[a-jackson]

Issue #, if available: #277

Description of changes:

If repsonse.ContentEncoding is not set then this will fallback to UTF8 to prevent an exception.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[srprash]

Hi @a-jackson
Thanks for opening this PR. Can you add a corresponding unit test for the fix too?

[a-jackson]

Hi @srprash

Just managed to get back to this, there doesn't appear to be any unit tests for the EC2Plugin at all right now and I'm not really sure how I'd do it given that is sends Http requests to the hard coded metadata endpoint.

[srprash]

Hi @a-jackson
The unit tests for EC2 plugin are here. The tests use a mocked EC2Plugin to return some predefined values. Can you give the tests a look and see if you can incorporate the new unit test for this change without a lot of effort?

[a-jackson]

I'm not really sure what you're after with that? If it's using a mocked EC2Plugin then it isn't going to be executing the code I have changed.

[srprash]

Ah I see. We mock the DoRequest method from the plugin so it will be difficult to test the change without having to re-structure the existing unit tests.
Given that this is a very minor change to add more code robustness and the unit testing effort is comparatively large, I think it is acceptable to merge this PR as is.

Thanks @a-jackson for contributing!