fix is_numeric/is_integer when checking non-string parameters

I expect a function called "is_numeric" or "is_integer" to check if a
variable is an integer or a number even if the variable passed by isn't
a string nor a number at all. Otherwise we should call them
is_string_a_number and is_string_an_integer and we have then to remove
the check for .is_a?(Number) and .is_a?(FixNum)