toJson() truncates some doubles to integers

[nyh]

The toJson() function, which is supposed to print columns of various types in JSON format, appears to print some - but not all - values of type double as integers. The following test (which has d as a double column) fails in a very curious way:

def test_tojson_double(cql, table1):
    p = random.randint(1,1000000000)
    stmt = cql.prepare(f"INSERT INTO {table1} (p, d) VALUES (?, ?)")
    cql.execute(stmt, [p, 123.456])
    assert list(cql.execute(f"SELECT d from {table1} where p = {p}")) == [(123.456,)]
    assert list(cql.execute(f"SELECT toJson(d) from {table1} where p = {p}")) == [("123.456",)]
    cql.execute(stmt, [p, 123123.123123])
    assert list(cql.execute(f"SELECT d from {table1} where p = {p}")) == [(123123.123123,)]
    assert list(cql.execute(f"SELECT toJson(d) from {table1} where p = {p}")) == [("123123.123123",)]

The curious part is that the first half of the test succeeds - the number 123.456 is correctly converted to the JSON "123.456". But then, 123123.123123 is incorrectly converted to just "123123", with no decimal point! (the order of the inserts isn't what matters, 123123.123123 fails even if it the only insert done).

The same test passes on Cassandra, so it's not an intended behavior (obviously).

This bug is reproduced by the following two xfailing tests:

1. test_json.py::test_tojson_double - which is the same test as shown above.
2. cassandra_tests/validation/entities/json_test.py::testToJsonFct, a translated Cassandra test which led me to this bug (it happens to this constant 123123.123123 in one of its many checks).