Merge pull request #28891 from sodabrew/patch-1

Remove mysql2 database adapter default username root

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -10,8 +10,6 @@ module ConnectionHandling # :nodoc:
     # Establishes a connection to the database that's used by all Active Record objects.
     def mysql2_connection(config)
       config = config.symbolize_keys
-
-      config[:username] = "root" if config[:username].nil?
       config[:flags] ||= 0
 
       if config[:flags].kind_of? Array