Fix #158 feature request (remove empty search params from sort_link params hash and URL)

Builds on the discussion in PR #207.

Submitted as a pull request to see if there is interest in having a shorter sort_link URL. 

If yes, will need some test coverage.
No need for nil or presence checking
Merge pull request #270 from ernie/fix-#158

Fix #158 feature request (remove empty search params from sort_link)