Suppress finaliser in Dispose if there's a protected Dispose(bool)

Fixes #746

Src/Newtonsoft.Json.Tests/JsonTextReaderTest.cs

@@ -3520,6 +3520,77 @@ null//comment
             Console.WriteLine("Done");
         }
 #endif
+        [Test]
+        public void DisposeSupressesFinalization()
+        {
+            UnmanagedResourceFakingJsonReader.CreateAndDispose();
+            GC.Collect();
+            GC.WaitForPendingFinalizers();
+            Assert.AreEqual(1, UnmanagedResourceFakingJsonReader.DisposalCalls);
+        }
+    }
+
+    public class UnmanagedResourceFakingJsonReader : JsonReader
+    {
+        public static int DisposalCalls;
+
+        public static void CreateAndDispose()
+        {
+            ((IDisposable)new UnmanagedResourceFakingJsonReader()).Dispose();
+        }
+
+        public UnmanagedResourceFakingJsonReader()
+        {
+            DisposalCalls = 0;
+        }
+
+        protected override void Dispose(bool disposing)
+        {
+            base.Dispose(disposing);
+            ++DisposalCalls;
+        }
+
+        ~UnmanagedResourceFakingJsonReader()
+        {
+            Dispose(false);
+        }
+
+        public override bool Read()
+        {
+            throw new NotImplementedException();
+        }
+
+        public override byte[] ReadAsBytes()
+        {
+            throw new NotImplementedException();
+        }
+
+        public override DateTime? ReadAsDateTime()
+        {
+            throw new NotImplementedException();
+        }
+
+#if !NET20
+        public override DateTimeOffset? ReadAsDateTimeOffset()
+        {
+            throw new NotImplementedException();
+        }
+#endif
+
+        public override decimal? ReadAsDecimal()
+        {
+            throw new NotImplementedException();
+        }
+
+        public override int? ReadAsInt32()
+        {
+            throw new NotImplementedException();
+        }
+
+        public override string ReadAsString()
+        {
+            throw new NotImplementedException();
+        }
     }
 
     public class ToggleReaderError : TextReader

Src/Newtonsoft.Json/JsonReader.cs

@@ -1085,6 +1085,7 @@ namespace Newtonsoft.Json
         void IDisposable.Dispose()
         {
             Dispose(true);
+            GC.SuppressFinalize(this);
         }
 
         /// <summary>