Fix indentation in docs
Tweak the JSON_DECODE_INT_AS_REAL test introduced in #123

Only run the imprecision part if json_int_t is long long, otherwise
the imprecision cannot be tested against json_int_t.

Also, convert the double value to json_int_t before checking the
imprecision, because otherwise the json_int_t is converted to double
and the imprecision happens in the "wrong direction".