Add find_index, has, reject array filters

[brunodccarvalho]

Hello hello, our friends over at shopify liquid shipped some new collection filters recently, available in their themes, I would assume mirroring these here would be of interest.

There are several compatibility issues whose resolution is not obvious. Most concern how null,undefined,blank,empty are handled when given (or not given) as the property value to these functions, will elaborate in the pull request.