Fix ArgumentError when S3 responds with invalid Expires Header

[trevorrowe]

Resolved an issue with Aws::S3::Client#head_object and #get_object where an ArgumentError was raised if Amazon S3 responded with an Expires header that contained an unparsable string.

The #head_object and #get_object response now return nil when the Expires header contains an invalid value. You can now access the raw string value of the Expires header with #expires_string.

# If Amazon S3 responds with `Expires: abc` as a header
resp = s3.head_object(bucket:'bucket', key:'key')
resp.expires #=> nil
resp.expires_string #=> "abc"

See related GitHub issue #1184.

[awood45]

Technically this will make it show up BEFORE 'Expires' in my testing, not after.

[awood45]

There's a mismatch in the comments about where ExpiresString goes in the hash order and where it is actually put. I'm not really sure why it matters though, at least in the context of the diff.

If we are deciding to care about this, we should probably add an assertion in the unit test that this order is ensured.

[trevorrowe]

I've confirmed that the order does not matter, but I've pushed a commit that makes sure they line up.

[awood45]

LGTM then