Improving theme data gathering on Update

Resolves #241 where @fjarrett reported some notices on the theme update page.

The source of the problem there was that the information can be stored in another variable called `theme_info` and it is a instance of [WP_Theme](http://codex.wordpress.org/Class_Reference/WP_Theme) which the methods change a bit from what was used.

I've included a conditional to check for the `theme` variable them if is not set I work with the `WP_Theme` instance.