fix a bug for browsers which are not based on webkit rendering engine.

### bug discription
the `header.innerHTML` should be changed to `this.header.innerHTML` in line 1683, it will throw an error which **header is not defined** if you are not on Webkit, because the variable `header` hasn't be defined at all, the Webkit provides a shortcut for `document.getElementById(someId)`, that is `someId`, so, you're accidently correct on Webkit, if not, the code will break.

### how to reproduce?
test on any platforms rather that Webkit.

### how to fix?
changed `header.innerHTML` to `this.header.innerHTML` in line 1683.
Merge pull request #497 from LeoYuan/patch-4

fix a bug for browsers which are not based on webkit rendering engine.