tests, pluralize singular table name.

This solves the following error:

     ActiveRecord::StatementInvalid: Could not find table 'guitars'

It seems that the table structure of the `Guitar` model has not been
necessary until now. Due to the wrong table name the model was not
correctly linked to the table.

activerecord/test/schema/schema.rb

@@ -356,7 +356,7 @@ def except(adapter_names_to_exclude)
     t.column :key, :string
   end
 
-  create_table :guitar, force: true do |t|
+  create_table :guitars, force: true do |t|
     t.string :color
   end