Should not validate that ComplexType#custom is a boolean

I'm fairly new to breeze, so this is likely a lack of understanding on my part, but I'm having trouble with the `ComplexType` constructor trying to validate that the `custom` attribute is a Boolean.

The docs above the constructor and other instances of the `custom` attribute indicate that it should be an object.

I put together a rudimentary pull request to correct this behavior, assuming it is incorrect, but I have not run any unit tests against the change.