allOf support

[jkone27]

i noticed many time in genration obj type is used instead of proper one in case of oneOf, maybe just the first json variant should be considered? in my case even with just 1 case of allOf or oneOf the method would not show anything other than (obj) in the method signature

notWorking:
          allOf:
            - $ref: '#/components/schemas/SomeType'
          nullable: true
working:
          $ref: '#/components/schemas/SomeType'

https://swagger.io/docs/specification/v3_0/data-models/oneof-anyof-allof-not/

[sergey-tihon]

Should be already fixed here #279 (comment)