Fixed Mention prefix detection

src/Discord.Net.Commands/Extensions/MessageExtensions.cs

@@ -29,10 +29,16 @@
 
             int endPos = text.IndexOf('>');
             if (endPos == -1) return false;
+            if (text.Length < endPos + 2 || text[endPos + 1] != ' ') return false; //Must end in "> "
 
             ulong userId;
-            if (!MentionUtils.TryParseUser(text.Substring(0, endPos), out userId)) return false;
-            return userId == user.Id;
+            if (!MentionUtils.TryParseUser(text.Substring(0, endPos + 2), out userId)) return false;
+            if (userId == user.Id)
+            {
+                argPos = endPos + 2;
+                return true;
+            }
+            return false;
         }
     }
 }