Spells: Regaining concentration after stun

[iiNGloriouS]

Past few weeks of playing my bard when I stun a target, they've been able to regain concentration and finish the cast. This shouldn't be happening, also seen it with kick stuns and T-staff procs.

Stuns should stop the cast entirely.

[Sunderfury]

Resolving due to age and lack of cited sources.