[CLA] [patch] dojo.substitute() returns undefined for unknown place holders

[wolfram]

Currently dojo.string.substitute works like this:

    >>> dojo.string.substitute("${i} is not defined, but ${x} is", {x:"'this'"});
    undefined

but it should do this imho

    >>> dojo.string.substitute("${i} is not defined, but ${x} is", {x:"'this'"});
    ${i} is not defined, but 'this' is

My usecase: I am doing replacement in multiple steps and want to replace only when the right data are given.