Fix language prop being ignored

[barrycohen]

Fixes a logic error which causes the language prop to be ignored unless autodetection is explicit disabled.

For example, in 2.1.2, the following is highlighted as cpp rather than shell, even though we have asked for shell:

<highlightjs language="shell" code="#include <iostream>"></highlightjs>

I've tested the fix over a wide range of cases. The example code I used is

#include <iostream>; import os

because it is autodetected as stan but renders differently for stan, cpp and shell, which means you can visually tell how it has been highlighted.

I believe the fix provides reasonable results in all cases.

Fixes #41.

[joshgoebel]

Awesome, also will need to update the CHANGELOG.

[barrycohen]

Good point, thanks. ChangeLog updated. I've assumed the change will go into 2.1.3.