Fix: model with required field that defaults to 0 can't be saved

Number.prototype.validateRequiredInput() merely checked if the getter
for the field returned a truthy value. That fails if the default happens
to be zero (which is a falsy value).

The fix is to simply check if the return value is a number.

fields/types/number/NumberType.js

@@ -39,7 +39,7 @@ number.prototype.validateInput = function (data, callback) {
 number.prototype.validateRequiredInput = function (item, data, callback) {
 	var value = this.getValueFromData(data);
 	var result = !!(value || typeof value === 'number');
-	if (value === undefined && item.get(this.path)) {
+	if (value === undefined && typeof item.get(this.path) === 'number') {
 		result = true;
 	}
 	utils.defer(callback, result);