If you put 3 wither skeleton skulls next to each other, then place a t of soul sand under them AND 3 blocks to the right, the 4th head you place will trigger the wither spawning.

S=Soul sand

H=Head

1=1st

2=2nd

etc

H1 H1 H1    H3

S2 S2 S2       S1 S1 S1              =Wither Spawning

     S2                 S1                     This graphic may view better on desktop

I have uploaded a YouTube video detailing what happened, it is possible to replicate it.  Works for North and East, but not South nor West.  I will upload both a screenshot of Before/After and the video aforementioned.  It spawns the wither, when it should not.  It seems to spawn in-between the two T's of Soul Sand, but the first patch is destroyed, so it is hard to tell.  

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Video:https://youtu.be/hNVsizDqBUA