I do not think `sharedTypes` should be a string. 

I know the type is declared and handled as a string multiple places but in [the only place I can find where the option is used](https://github.com/wsdl2phpgenerator/wsdl2phpgenerator/blob/master/src/Wsdl2PhpGenerator/Generator.php#L326), it is used as a boolean.

I know you changes did not mean to address this issue here but changing from `false` to `null` only makes things worse. 

As I see it either we should change the type to boolean in this pull request or revert this line and create a separate issue for the problem.