Fix Deps's broken _assign (harmless)

You can't do ({hasOwnProperty: 12345}).hasOwnProperty("hasOwnProperty") -- think about it.

Basically, `obj.hasOwnProperty(key)` is considered harmful.  `Object.prototype.hasOwnProperty.call(obj, key)` is fine.  We should standardize on `_.has` or something and do a pass through the codebase.

This is my code so my bad.
Don't use _.pick in observe-sequence

Trying to stick to a small set of _ methods for stand-alone Blaze