Json writer, only emit floating point finite numbers, ignore INF/NaN

My interpretation of RFC4627, Section 2.4 "Numbers" of:
  Numeric values that cannot be represented as sequences of digits
   (such as Infinity and NaN) are not permitted.

I have also verified this matter with NodeJS JSON.stringify() that
emitting a null is consistent behavior with a JSON implementation
written in JavaScript.

Previously Qt would emit:
{
 plusInfinity: inf,
 minusInfinity: -inf,
 notANumber: nan
}

Which maybe turned into a string values of "inf", "-inf", "nan" by
the receiving parser.  Now it returns the JSON null value just like
NodeJS JSON.stringify().

Change-Id: I9f9c17f12b2606280806c47a9d90465c4ba5f786
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/json/qjson_p.h

@@ -60,6 +60,7 @@
 #include <qatomic.h>
 #include <qstring.h>
 #include <qendian.h>
+#include <qnumeric.h>
 
 #include <limits.h>
 

src/corelib/json/qjsonwriter.cpp

@@ -169,9 +169,14 @@ static void valueToJson(const QJsonPrivate::Base *b, const QJsonPrivate::Value &
     case QJsonValue::Bool:
         json += v.toBoolean() ? "true" : "false";
         break;
-    case QJsonValue::Double:
-        json += QByteArray::number(v.toDouble(b));
+    case QJsonValue::Double: {
+        const double d = v.toDouble(b);
+        if (qIsFinite(d))
+            json += QByteArray::number(d);
+        else
+            json += "null"; // +INF || -INF || NaN (see RFC4627#section2.4)
         break;
+    }
     case QJsonValue::String:
         json += '"';
         json += escapedString(v.toString(b));

tests/auto/corelib/json/tst_qtjson.cpp

@@ -44,6 +44,7 @@
 #include "qjsonobject.h"
 #include "qjsonvalue.h"
 #include "qjsondocument.h"
+#include <limits>
 
 #define INVALID_UNICODE "\357\277\277" // "\uffff"
 #define UNICODE_DJE "\320\202" // Character from the Serbian Cyrillic alphabet
@@ -94,6 +95,7 @@ private Q_SLOTS:
     void toVariantList();
 
     void toJson();
+    void toJsonSillyNumericValues();
     void fromJson();
     void fromJsonErrors();
     void fromBinary();
@@ -1080,6 +1082,34 @@ void tst_QtJson::toJson()
     }
 }
 
+void tst_QtJson::toJsonSillyNumericValues()
+{
+    QJsonObject object;
+    QJsonArray array;
+    array.append(QJsonValue(std::numeric_limits<double>::infinity()));  // encode to: null
+    array.append(QJsonValue(-std::numeric_limits<double>::infinity())); // encode to: null
+    array.append(QJsonValue(std::numeric_limits<double>::quiet_NaN())); // encode to: null
+    object.insert("Array", array);
+
+    QByteArray json = QJsonDocument(object).toJson();
+
+    QByteArray expected =
+            "{\n"
+            "    \"Array\": [\n"
+            "        null,\n"
+            "        null,\n"
+            "        null\n"
+            "    ]\n"
+            "}\n";
+
+    QCOMPARE(json, expected);
+
+    QJsonDocument doc;
+    doc.setObject(object);
+    json = doc.toJson();
+    QCOMPARE(json, expected);
+}
+
 void tst_QtJson::fromJson()
 {
     {