JCLOUDS-630: Fixes Swift object expiration mapping

This PR addresses [JCLOUDS-630](https://issues.apache.org/jira/browse/JCLOUDS-630)

- Map the HTTP `expires` field accessed via `Payload.getContentMetadata().getExpires()` to the Swift `X-Delete-At` header for object expiration.
- Prefer `HashCode` to `byte[]` MD5
- Removed unused `ToSwiftObject` class
- Updated Mock/Live Tests
How, if at all, are we actually testing expiration here? From what I can understand, we're simply checking whether a call with "expires" set doesn't fail..?