Light dawns; symbolic keys are converted to strings.

Still not really any closer to repeating/repairing the exact failure that led me to file Issue #62, and in hindsight not all that surprising, *but* all hash keys in hashes used as Mutations parameters are represented in the `filtered_inputs` as *strings* if they were originally passed in as *symbols*. Since HTTP doesn't know diddly about Ruby symbols, conversion to strings is blindingly obvious.

But I'm still trying to repeat my initial failure in a test.