CASE with alias is not properly parsed

Feeding this SQL to the Parser:

SELECT
  CASE 
    WHEN a.kol = 1 THEN 'one'
    ELSE a.kol
  END as 'case'
FROM foo a;

will result in errors, and calling build() on the statement will result in this SQL:

SELECT CASE 'case' ELSE a.kol AS `END` END FROM foo AS `a`

The errors array contains these errors:

• An alias was previously found.
• An alias was expected.
• Unexpected keyword.
• Unexpected end of CASE expression

I encountered this problem since I upgraded from 5.8.2 to 5.9.1.

Empieza reproduciendo la expresión CASE proporcionada a través del Parser y comparando los errores reportados y la salida de build(). Traza cómo las expresiones CASE consumen alias y palabras clave, y después añade un caso de regresión que muestre que este SQL se analiza sin errores y que build() se genera con el alias preservado correctamente.