Fix model sanitisation

[macisamuele]

The goal of this PR is to address a problem, discovered internally, related to the too aggressive approach during sanitisation.

Model named as "model with spaces" or "model.with.dots" could lead to inconsistent names.
The updated approach does avoid the split of the model name but instead does replace the "bad" characters with something (an underscore) that will force a valid PascalCase representation and will be removed by sanitizeKotlinSpecificNames call.

---

Internally we had something similar to

... # this is only a snipped of the specs
definitions:
  a:
    type: object
    x-model: a
  wrapper:
    a:
      additionalProperties:
        $ref: '#/definitions/a'
      type: object
      x-model: wrapper.a

Without the fix the generated code ignores the fact that #/definitions/wrapper/properties/a should be a Map<String, A> and it generates as A and this is bad as it changes the semantic of the endpoint and potentially makes parsing the response crashing as incompatible.

With the fix the generated code for #/definitions/wrapper/properties/a will be WrapperA (a type alias to Map<String, A>) which is what we would expect.

[cortinico]

LGTM :)