Stop Plenisher updating source block of same type. (Better fix for issue #1589)

Stops Fluid Plenishers wasting fluids by replacing existing source
blocks.
For example, if it will not replace existing lava source blocks if it is
placing new lava. This means when you reset its' calculations it wont
re-fill your entire lake, it will only fill non-source blocks.
This should solve the complication in the previous commit which 'fixed'
issue #1589.