-Fix bug when JValue of type string has a null string value

-Fix JToken casting bugs to float and decimal

Src/Newtonsoft.Json.Tests/Linq/JObjectTests.cs

@@ -410,5 +410,23 @@ Parameter name: arrayIndex")]
         i++;
       }
     }
+
+    [Test]
+    public void WriteObjectNullStringValue()
+    {
+      string s = null;
+      JValue v = new JValue(s);
+      Assert.AreEqual(null, v.Value);
+      Assert.AreEqual(JTokenType.String, v.Type);
+
+      JObject o = new JObject();
+      o["title"] = v;
+
+      string output = o.ToString();
+      
+      Assert.AreEqual(@"{
+  ""title"": null
+}", output);
+    }
   }
 }

Src/Newtonsoft.Json.Tests/Linq/JTokenTests.cs

@@ -235,6 +235,11 @@ namespace Newtonsoft.Json.Tests.Linq
       Assert.AreEqual(ulong.MaxValue, (ulong)new JValue(ulong.MaxValue));
       Assert.AreEqual(ulong.MaxValue, (ulong)new JProperty("Test", new JValue(ulong.MaxValue)));
       Assert.AreEqual(null, (string)new JValue((string)null));
+      Assert.AreEqual(5m, (decimal)(new JValue(5L)));
+      Assert.AreEqual(5m, (decimal?)(new JValue(5L)));
+      Assert.AreEqual(5f, (float)(new JValue(5L)));
+      Assert.AreEqual(5f, (float)(new JValue(5m)));
+      Assert.AreEqual(5f, (float?)(new JValue(5m)));
     }
 
     [Test]

Src/Newtonsoft.Json/Linq/JToken.cs

@@ -517,7 +517,7 @@ namespace Newtonsoft.Json.Linq
       if (v == null || !ValidateFloat(v, true))
         throw new ArgumentException("Can not convert {0} to Decimal.".FormatWith(CultureInfo.InvariantCulture, GetType(v)));
 
-      return (decimal?)v.Value;
+      return (v.Value != null) ? (decimal?)Convert.ToDecimal(v.Value) : null;
     }
 
     /// <summary>
@@ -613,7 +613,7 @@ namespace Newtonsoft.Json.Linq
       if (v == null || !ValidateFloat(v, true))
         throw new ArgumentException("Can not convert {0} to Single.".FormatWith(CultureInfo.InvariantCulture, GetType(v)));
 
-      return (float?)v.Value;
+      return (v.Value != null) ? (float?)Convert.ToSingle(v.Value) : null;
     }
 
     /// <summary>
@@ -627,7 +627,7 @@ namespace Newtonsoft.Json.Linq
       if (v == null || !ValidateFloat(v, false))
         throw new ArgumentException("Can not convert {0} to Decimal.".FormatWith(CultureInfo.InvariantCulture, GetType(v)));
 
-      return (decimal)v.Value;
+      return Convert.ToDecimal(v.Value);
     }
 
     /// <summary>

Src/Newtonsoft.Json/Linq/JValue.cs

@@ -264,15 +264,14 @@ namespace Newtonsoft.Json.Linq
       }
     }
 
-    private static void WriteConvertableValue(JsonWriter writer, IList<JsonConverter> converters, Action<object> _defaultWrite, object value)
+    private static void WriteConvertableValue(JsonWriter writer, IList<JsonConverter> converters, Action<object> defaultWrite, object value)
     {
       JsonConverter matchingConverter;
 
-      JsonSerializer.HasMatchingConverter(converters, value.GetType(), out matchingConverter);
-      if (matchingConverter != null)
+      if (value != null && JsonSerializer.HasMatchingConverter(converters, value.GetType(), out matchingConverter))
         matchingConverter.WriteJson(writer, value);
       else
-        _defaultWrite(value);
+        defaultWrite(value);
     }
 
     /// <summary>
@@ -294,7 +293,7 @@ namespace Newtonsoft.Json.Linq
           WriteConvertableValue(writer, converters, v => writer.WriteValue(Convert.ToDouble(v, CultureInfo.InvariantCulture)), _value);
           break;
         case JTokenType.String:
-          WriteConvertableValue(writer, converters, v => writer.WriteValue(v.ToString()), _value);
+          WriteConvertableValue(writer, converters, v => writer.WriteValue((v != null) ? v.ToString() : null), _value);
           break;
         case JTokenType.Boolean:
           WriteConvertableValue(writer, converters, v => writer.WriteValue(Convert.ToBoolean(v, CultureInfo.InvariantCulture)), _value);