FIXING SES get_identity_dkim_attributes fails when input length > 1

boto.ses.connection::get_identity_dkim_attributes would call
_make_request() but would incorrectly parse the XML response from AWS.
If an input list with length greater than 1 was provided, this method
would only return a response for a single list element.

Adding DkimAttributes to list_markers and updating unit tests.
Merge pull request #2810 from monkeysecurity/develop

Fix SES get_identity_dkim_attributes when input length > 1. Fixes #2810.