NameError#missing_name? can jsut use NameError#name if the arg is a Symbol

NameError#name returns a missing name as a symbol, so if the given name
is a symbol, it doesn't have to use #missing_name to get the last constant
name in the error message.

activesupport/lib/active_support/core_ext/name_error.rb

@@ -23,8 +23,7 @@ def missing_name
   #   # => true
   def missing_name?(name)
     if name.is_a? Symbol
-      last_name = (missing_name || '').split('::').last
+      self.name == name
-      last_name == name.to_s
     else
       missing_name == name.to_s
     end