fix problem of atan function when PARSE2NUMBER is true

[yosuke]

This patch will fix the minor problem exist in atan function, which returns undefined variable when PARSE2NUMBER is true and argument is variable.

Before:

> nerdamer.getCore().Settings.PARSE2NUMBER = true;
> nerdamer('atan(x)').text();
TypeError: Cannot read property 'text_' of undefined
    at Expression.text (/home/yosuke/nerdamer/nerdamer.core.js:1853:28)

After:

> nerdamer.getCore().Settings.PARSE2NUMBER = true;
> nerdamer('atan(x)').text()
'atan(x)'

[jiggzson]

@yosuke, thanks!