Use User::getDefaultOption() instead of $wgDefaultUserOptions

Use User::getDefaultOption() instead of $wgDefaultUserOptions

Using User::getDefaultOption() in User::setOption() sets preferences like
'language' to the right default value, whereas using $wgDefaultOptions
sets the language to null because language is not part of $wgDefaultUserOptions,
but it is part of return value of User::getDefaultOption().

This only makes problems when reusing the same user object, because when
a new user object is created, the value of 'language' is correct.

Change-Id: I43a32c66fc3997a4f842c63af374e84d234602b2