Make InvalidProtocolBufferException.InvalidTag internal.

We don't need to expose the InvalidProtocolBufferException factory method now that the generated code doesn't throw the exception.
diff --git a/csharp/src/Google.Protobuf/InvalidProtocolBufferException.cs b/csharp/src/Google.Protobuf/InvalidProtocolBufferException.cs
index bbba9b3..6905a6a 100644
--- a/csharp/src/Google.Protobuf/InvalidProtocolBufferException.cs
+++ b/csharp/src/Google.Protobuf/InvalidProtocolBufferException.cs
@@ -70,7 +70,7 @@
         /// <summary>

         /// Creates an exception for an error condition of an invalid tag being encountered.

         /// </summary>

-        public static InvalidProtocolBufferException InvalidTag()

+        internal static InvalidProtocolBufferException InvalidTag()

         {

             return new InvalidProtocolBufferException(

                 "Protocol message contained an invalid tag (zero).");