Don't type cast non-sensible values

[voxpelli]

Fixes #186 by checking if it's a sensible value before it casts it, so that non-sensible values won't be casted to eg. a string

[ljharb]

I'd need to see some test cases that cover this new behavior.

What is a "non-sensible" value? As I mentioned in #186, values sent over the wire are only ever strings, so it's a bit strange to allow a non-serializable value.