Fix issues with DefaultRunMode

For a command to use the DefaultRunMode, it must now have it's RunMode set to RunMode.Default (this is the default value on CommandAttribute now).

src/Discord.Net.Commands/Attributes/CommandAttribute.cs

@@ -6,7 +6,7 @@ namespace Discord.Commands
     public class CommandAttribute : Attribute
     {
         public string Text { get; }
-        public RunMode RunMode { get; set; }
+        public RunMode RunMode { get; set; } = RunMode.Default;
 
         public CommandAttribute()
         {

src/Discord.Net.Commands/Builders/CommandBuilder.cs

@@ -17,7 +17,7 @@ namespace Discord.Commands.Builders
         public string Name { get; set; }
         public string Summary { get; set; }
         public string Remarks { get; set; }
-        public RunMode? RunMode { get; set; }
+        public RunMode RunMode { get; set; }
         public int Priority { get; set; }
 
         public IReadOnlyList<PreconditionAttribute> Preconditions => _preconditions;

src/Discord.Net.Commands/Info/CommandInfo.cs

@@ -37,7 +37,7 @@ namespace Discord.Commands
             Summary = builder.Summary;
             Remarks = builder.Remarks;
 
-            RunMode = builder.RunMode ?? service._defaultRunMode;
+            RunMode = (builder.RunMode == RunMode.Default ? service._defaultRunMode : builder.RunMode);
             Priority = builder.Priority;
             
             if (module.Aliases.Count != 0)

src/Discord.Net.Commands/RunMode.cs

@@ -2,6 +2,7 @@
 {
     public enum RunMode
     {
+        Default,
         Sync,
         Mixed,
         Async