Lisp RPCQ balks on serializing rationals living in float slots

[ecpeterson]

Lisp won't prevent you from storing a rational into a slot marked for floats, but when you go to serialize the object, it'll throw an exception. Either messages should be type-safe enough to catch this error at assignment time (or at compile time!), or rationals should be coerced into floats before handoff to cl-messagepack.