Fix "cssID" attribute in shortcodes

"cssID" attribute in shortcode wasn't working.
From codex for "add_shortcode" function: "Shortcode attribute names are always converted to lowercase before they are passed into the handler function. Values are untouched."
Therefore "rpwe_shortcode" function that is used in shortcode.php gets $atts array with lowercased attribute names.
And "rpwe_get_default_args" function returns array with key named 'cssID'. Due to the fact that key names are case sensitive, cssID remains empty.