[BUG][Ruby Client] The client library does not use etag and If-none-match to reduce network payload

Describe the bug
The client library does not use etag and if-none-match to reduce network payload.

To Reproduce
Steps to reproduce the behavior:

1. Run a sample Ruby application that uses the Singleton Ruby client library in online mode.
2. Trigger a an action that sends a request to the Singleton service to fetch data and load data in cache.
3. Notice that the Singleton service responds with an HTTP 200 OK, with an eTag header.
4. After the data in cache has expired, trigger the same action.
5. Error: Notice that the HTTP request does not include an If-none-match header.

Expected behavior
The HTTP request in step 5 should have included an If-none-match header with value the same as the value of the eTag header in Step 3.

Additional context
https://confluence.eng.vmware.com/display/GQ/Etag+and+Cache-Control