Add safety in case getSource was called

[mpchadwick]

Per #32 usesSource() may incorrectly return true if getSource() was previously called on the attribute.

If we didn't get the value the first time around (perhaps due to the getSource issue) we try to fetch again using the mechanics that would be used for attributes without source models.

[Vinai]

Thanks for the PR! The additional check if $value is true would fail for empty strings or strings starting with a "0".
How about changing the check for usesSource() to check if the frontend input is select or multiselect?

[mpchadwick]

@Vinai updated per your feedback.

[mpchadwick]

@Vinai don't mean to be a pest, but I'm trying to install this on one of our sites, but it won't work until this issue is resolved.

[Vinai]

Thanks for your effort!