Add Prop.type: Function behavior

[pikax]

When the prop has the type of Function, the default is treated as a value instead of factory:
https://github.com/vuejs/docs-next/blob/master/src/guide/component-props.md#prop-validation

    // Type function, the default will be the default function
    propG: {
      type: Function,
      default: function () {
        return 'Default function'
      },
    },

PR with the change vuejs/core#1349

[NataliaTepluhina]

Closing as related PR was merged 🎉