Fix HttpFeed so again reuses polls

- If two sensors make the same http request (with just
  different post-processing on the result) then just poll
  once and pass the result to each.

  Was broken by switching to Supplier<URI> instead of just
  a simple URI, so hashcode/equals failed.
Merge pull request #652 from aledsage/fix/HttpFeed-reuses-polls

Fix HttpFeed so again reuses polls