Add Header Options to AWS::S3::Client#head_object

The #head_object operation is supposed to parallel the #get_object
operation for all input headers, but this was not the case due to an
omission in the hand coding. This change adds the remaining input
headers and the associated test.

Resolves issue #495.
Merge pull request #518 from aws/issue495

Add Header Options to AWS::S3::Client#head_object