Fix the xpath_query mistake for element class

`//*[contains(@class, "target_class")]//*/text()` may cause mistakes in cases like this:

`class="foo_bar"` and `pangu.element_spacing("foo")`.

In fact, it also causes some strange mistakes. When I use `pangu.element_spacing` for a class query, it will always return `false`. I put the sample at [http://jsfiddle.net/shuding/od99z6bn/](http://jsfiddle.net/shuding/od99z6bn/).

BTW, I really like your great plugin! : )