Fixed "1 million" number to words in Spanish

The "1 million" in letters were wrong in Spanish. It is "un millón" not just simply "millón"
Source: http://es.wikipedia.org/wiki/Anexo:Nombres_de_los_n%C3%BAmeros_en_espa%C3%B1ol

With this PR I finish my review on NumberToLetters in Spanish.

@MehdiK @thunsaker Maybe I'll sound quite purist but we are not validating if we are processing correctly the conversion. For instance: In English the output would be wrong for numbers above (or equal than) a trillion and in Spanish the output would be wrong with numbers above (or equal than) a billion.
If we don't want to waste our time coding for big (and unusual) numbers, we should throw and exception at least.