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 module is that `transform` and `default` should return `undefined` to disallow merging.

My use case for using object-mapper as co convert object properties from one storage to another, preserving values which is defined but can coerced to `false`.
0.0.5