There was no fix, until now:
You will need another player's save files for this fix. The file names are:
- savefile.foo
- savefile.svg
If you cannot get the files from a friend, you can DOWNLOAD THEM FREE BELOW:

This set has all the missions (but not all the weapons) unlocked. The story info for the save file is for the "happiest" ending.
You will need to be modifying files for this fix, so DO SO AT YOUR OWN RISK!!!
I am not responsible for any negative consequences you incur upon yourself and others for performing this fix.
Since you will be replacing the save files, you will loose any mission progress, unlocked weapons, perks, etc., and storyline info in your previous save file. But you will keep all achievements since they are stored on Steam's servers.