Encode reals correctly

This patch changes the sprintf format from "%0.17f" to "%.17g", as the
f format specifier doesn't print the exponent at all. This caused
losing precision in all but the most simple cases.

Because the g specifier doesn't print the decimal fraction or exponent
if they're not needed, a ".0" has to be appended by hand in these
cases. Otherwise the value's type changes from real to integer when
decoding again.

Thanks to Philip Grandinetti for reporting this issue.