Throw when attempting to modify a message not made by the current user (#992)

* Throw when attempting to modify a message not made by the current user

* Didn't realize the client is passed into the MessageHelper function

* Respond to feedback

src/Discord.Net.Rest/Entities/Messages/MessageHelper.cs

@@ -1,4 +1,4 @@
-using Discord.API.Rest;
+using Discord.API.Rest;
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
@@ -13,6 +13,9 @@ namespace Discord.Rest
         public static async Task<Model> ModifyAsync(IMessage msg, BaseDiscordClient client, Action<MessageProperties> func,
             RequestOptions options)
         {
+            if (msg.Author.Id != client.CurrentUser.Id)
+                throw new InvalidOperationException("Only the author of a message may change it.");
+
             var args = new MessageProperties();
             func(args);
             var apiArgs = new API.Rest.ModifyMessageParams

src/Discord.Net.Rest/Entities/Messages/RestUserMessage.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;

src/Discord.Net.WebSocket/Entities/Messages/SocketUserMessage.cs

@@ -1,4 +1,4 @@
-using Discord.Rest;
+using Discord.Rest;
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;