Fixed njs_value_to_string() with non-writable dst argument.

Fixed njs_value_to_string() with non-writable dst argument.

njs_arg(args, nargs, N) returns a pointer to Nth argument OR a pointer
to undefined constant value njs_value_undefined if N >= nargs.
njs_value_to_string() writes to a dst argument its result.
This means that it is incorrect to use value of njs_arg() directly
as a second argument to njs_value_to_string().

This closes #570 issue on Github.