Merge not-undefined values from one object to another

This will allow to merge values like empty string, null of false values. These values are valid and considered meaningful. The only change to semantics of the module is that `transform` and `default` should return `undefined` to disallow merging (previously it can be any value coerced to `false`).

My use case for using object-mapper is to convert objects from one storage to another, preserving values which is defined but can be `false` if coerced to boolean.