add a property to QJsonDocument for double precision

Hi,

As I can see in the current source then formatting double values to JSON text there is a following code:

QJsonWriter.cpp

case QJsonValue::Double: {
        const double d = v.toDouble(b);
        if (qIsFinite(d)) // +2 to format to ensure the expected precision
            json += QByteArray::number(d, 'g', std::numeric_limits<double>::digits10 + 2); // ::digits10 is 15
        else
            json += "null"; // +INF || -INF || NaN (see RFC4627#section2.4)
        break;
    } 

However there are many cases then server side expects a doubles with specified precision (most known example is Bitcoind RPC which expects 8 digits), with a current implementation its an absolute pain to implement JSON formatting with custom precision options.

I propose to add a setting in the formatter via QJsonDocument, how Doubles should be converted into the strings.

it can be done just by adding a parameter to 'toJson' function or by adding a new setting to QJsonDocument and use it in toJson call