Add the same debug check we do for integers to floats
Add the same debug check we do for integers to floats

This has the same reasoning. If you're using the `sql` function (meaning
you're manually marking the types), it's easy to mistake a function
which returns a double for one which returns a float. This assertion is
in debug mode only, as this problem does not affect our query builder
which enforces correctness in types.

mcasper/diesel,diesel-rs/diesel,cascalheira/diesel,diesel-rs/diesel,sgrif/diesel,sgrif/diesel,cascalheira/diesel,mcasper/diesel