Don't call retry if command prompt has default value.

discord-akairo#208

Author: Zendrex

src/struct/commands/arguments/Argument.js

@@ -120,7 +120,7 @@ class Argument {
         const commandDefs = this.command.argumentDefaults;
         const handlerDefs = this.handler.argumentDefaults;
         const optional = choice(
-            this.prompt && this.prompt.optional,
+            this.default ? true : this.prompt && this.prompt.optional,
             commandDefs.prompt && commandDefs.prompt.optional,
             handlerDefs.prompt && handlerDefs.prompt.optional
         );