Redis fixes and tests

Fixed:
* `processDoubleResponse()` Redis returns self::TYPE_BULK for double/float
* $cmd should always be lowercase in `client()` for $cmd == 'list' comparison
* `doEval` / `script` should return null on error
* `self::lastError` should be null by default
* Script `$subcmd` parameter should be lowercased for comparison.

Added:
* Tests for client LIST command
* Tests for `incr` `incrBy` `decr` `decrBy` `incrByFloat`
* Missing `processClientListResponse`
* Set `self::lastError` on script error and tests for this
* `script()` returns to be compatible with phpredis and tests for these.