Insertion into Boolean columns doesn't work

[roddyyaga]

On line 58 of query.mll, the function used to convert strings to Booleans Ppx_mysql_runtime.bool_of_string takes a string of an integer (e.g. "1" or "0") as input, which fits with how Booleans are stored in MySQL. But for converting Booleans to strings, Stdlib.string_of_bool is used, which converts false to "false" rather than "0". Therefore it is not possible to use ppx_mysql for Booleans.

Proposed fix:
In query.mll, replace Stdlib.string_of_bool with fun b -> if b then "1" else "0" or similar.