Our hash caching is now so aggressive that even if the precomputed hash was ignored, we'd still be pulling the hash out of the cache. So we now clear the cache to be sure that if the precomputed hash was ignored, that would trigger another call to `ComputeNode::hash()`.

…lue()`

And adjust `TypedPlugTest.testPrecomputedHash()` to actually test a TypedPlug instead of a StringPlug.