Multi-type prop validation [Boolean, String] cast empty string to `true`

[fliptheweb]

Vue.js version

2.0.2

Reproduction Link

https://jsfiddle.net/fliptheweb/zjr41svn/4/

Steps to reproduce

What is Expected?

An empty string '' must pass to the component like an empty string.

What is actually happening?

It's type cast type to boolean true

[fliptheweb]

It seems that if types validation includes String and Boolean together, we should not check here for the blank line.

There is another case when we have hyphenated string that looks like a prop.