Handle non string attribute/property value

[Javey]

Everyone is discussing serialization #20 #31 , it's complex. How about this simple PR?

Fixed:

1. Any attribute can be stringified if you set it by setAttribute. So :
   • div.setAttribute('data-null', null) should be serialized to <div data-null="null"></div> instead of <div data-null=""></div>
   • div.setAttribute('data-number', 0) should be serialized to <div data-number="0"></div> instead of <div data-number=""></div>
2. The false property should not be serialized. So:
   • option.selected = false should be serialized to <option></option> instead of <option selected=""></option>