Fix ignore `generator.platformClass` property
Fix ignore `generator.platformClass` property

`generator.platformClass` property, in the configuration file, is
always overwritten by the default value of `platform` option, in
`Propel\Generator\Command\AbstractCommand` class.
This commit fixes that behavior.

vdbf/Propel2,ktounet/Propel2,Big-Shark/Propel2,cristianoc72/Propel2,jeffblack360/Propel2,farhanrahman/Propel2,david0/Propel2,marcj/Propel2,SCIF/Propel2,propelorm/Propel2,jeremydenoun/Propel2,Big-Shark/Propel2,mhitza/Propel2,marcj/Propel2,marwein/Propel2,ikeyan/Propel2,jeremydenoun/Propel2,vdbf/Propel2,david0/Propel2,peter17/Propel2,pimpreneil/Propel2,ktounet/Propel2,jeffblack360/Propel2,pimpreneil/Propel2,farhanrahman/Propel2,kitsunde/Propel2,SwissalpS/Propel2,SCIF/Propel2,staabm/Propel2,staabm/Propel2,nibsirahsieu/Propel2,marwein/Propel2,peter17/Propel2,kitsunde/Propel2,ikeyan/Propel2,cristianoc72/Propel2,mhitza/Propel2,SwissalpS/Propel2,propelorm/Propel2,nibsirahsieu/Propel2