Fix ToJSON for PersistValue

[parsonsmatt]

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[parsonsmatt]

This is exactly the problem I was trying to solve, and exactly the problem that my patch introduced. Sigh. I'm disappointed that I didn't catch it.

So - the patch makes PersistDbSpecific a catch-all match, expanding to PersistLiteral_ _ bs. Which means that the PersistDbSpecific woudl encode all values with the p prefix! That's bad. This fixes that problem.