Fix the bug where reloading the page will turn the placeholder text into actual value of the filed

### Description

When doing page reloading (Ctrl+R in IE8, for example), since in https://github.com/jamesallardice/Placeholders.js/blob/master/lib/main.js#L102 the element value is set and not cleared during the reloading. So the second time https://github.com/jamesallardice/Placeholders.js/blob/master/lib/main.js#L100 will evaluate to false, thus no placeholder is rendered. Instead, the input will have the placeholder value as the actual value. This commit fixes that.