feat: add IsString validation to username field in Post class

Author: gabriel-logan

README.md

@@ -63,6 +63,7 @@ import {
   Length,
   IsEmail,
   IsNotBlank,
+  IsString,
   IsFQDN,
   IsDate,
   Min,
@@ -84,6 +85,7 @@ export class Post {
   @IsEmail()
   email: string;
 
+  @IsString()
   @IsNotBlank()
   username: string;