Fill in key name with boolean attribute

[kevin940726]

Currently if we need to parse the boolean attributes, we have to specify values of them with the attribute name.

<input required="required" />
// compiles to...
<input required />

In other words, this won't work...

<input required />
// compiles to...
<input />

I added a very simple logic to auto fill in the key name of the attribute to the value if the value is empty.

<input required />
// compiles to...
<input required="" />

[coveralls]

Coverage remained the same at 100.0% when pulling a423b83 on kevin940726:master into f9f4cca on mikenikles:master.

[aknuds1]

Looks good to me.