util: improve util.isDate() function
util: improve util.isDate() function

The old implementation was fragile. i.e. node-time is an example of a user-land
module that exports an extended Date object (with a few added functions on it's
own Date object's prototype). In that case, the old check fails.

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node