NEW: setDefaultLangs('auto') now check mysoc and conf

[atm-thomasb]

NEW|New setDefaultLangs now checks MAIN_LANG_DEFAULT and mysoc lang if 'auto'

Currently, setDefaultLangs with 'auto' argument sets the lang to the server's.
I've added so that it will beforehand check if $mysoc->defaultlang is defined and if yes it will take its lang,
otherwise if 'MAIN_LANG_DEFAULT' is defined it takes its lang.

Otherwise it does the same thing as before (server's lang).

[atm-thomasb]

Travis currently returns an error because it expects an output that changed with my PR:

$_SERVER['HTTP_ACCEPT_LANGUAGE'] = "' malicious text with quote";
$tmplangs->setDefaultLang('auto');
print METHOD.' $tmplangs->defaultlang='.$tmplangs->defaultlang."\n";
$this->assertEquals($tmplangs->defaultlang, 'malicioustextwithquote_MALICIOUSTEXTWITHQUOTE');

With the new system when given 'auto' it returns "en_US" instead. So it's as it's supposed to be, but is different from before.