Upsert

* Bug fixes

* Add parentheses to equality expressions

* Enhancements

* On Postgres, use (A = B OR (A IS NULL AND B IS NULL)) instead of A IS NOT DISTINCT FROM B because the latter does not use indexes
* pass :eager_nullify => false as the third argument to Upsertrow to disable clearing of null HStore keys

* Bug fixes

* Properly check for NULL equality when creating the UPSERT functions - thanks pnomolos - https://github.com/seamusabshere/upsert/issues/25
* When using Mysql2 client (MRI), don't pass timezone to DateTime columns - thanks kjeremy! - https://github.com/seamusabshere/upsert/issues/24

* Bug fixes

* Rookie mistake - gsub!'ed an input arg, sorry. Blew up on a frozen string, thank goodness.

* Breaking changes

* For Postgres Hstore, null keys are deliberately deleted - so there's no way to set "foo" => NULL - "foo" will just get deleted as a key
* Name merge functions "upsert1_2_0" instead of "upsert_"

* Breaking changes

* columns named "created_at" and "created_on" will only be set if it's a new row - thanks derekharmel! https://github.com/seamusabshere/upsert/pull/15

* Enhancements

* Detect invalid columns passed in either selector or setter - inspired by atandrau, thanks! https://github.com/seamusabshere/upsert/issues/18

* Bug fixes

* Always convert symbols to strings when used as bind vars - thanks towerhe! - https://github.com/seamusabshere/upsert/pull/16

* Enhancements

* :assume_function_exists option to avoid creating same merge function over and over
* Don't die on first occurrence of "tuple concurrently updated"