Fix: Model generation using oneOf, allOf & anyOf

[egormanga]

According to the JSON Schema spec, when a type definition has oneOf, allOf or anyOf variation, the variant definition might contain only specific fields, rather than a complete type or reference:

SomeType:
  type: object
  properties:
    x: {type: integer}
    y: {type: integer}
    z: {type: integer}
  required: [x]
  oneOf:
    - required: [y]
    - required: [z]

Currently, in such case the entire type fails to generate and gets treated as Any, resulting in lacking validation.

This implements the proper merging behavior, allowing to fully generate such types with their variants.

Additionally, replaces a relevant error-prone mutating operation, stumbled upon during debugging of this problem, with a non-mutating one due to unnecessity of the former.