@Pattern is not generated for scalar types

Hi.
Given the following definitions:

swagger: '2.0'
info:
  title: MyDomainObject
  version: "1.0"
definitions:
  MyDomainObject:
      type: object
      required:
        - currency
      properties:
        currency:
          $ref: '#/definitions/CurrencyIso'
        riskCurrency:
          $ref: '#/definitions/CurrencyIso'

  CurrencyIso:
    description: |
      The ISO currency definition.
    type: string
    pattern: ^[A-Z]{3}|[0-9]{3}$
    example: CHF
paths: {}

when the class is generated, no @Pattern annotation is inserted on the getter of the fields referencing the CurrencyIso.
I'm actually using the swagger-codegen-maven-plugin with version 2.3.1.
It seems that version 2.4.x fixes this issue, but then the patterns are not escaped when generated, leading to not compilable code in certain cases (e.g., if it contains \d+). There is already this issue about it.

Start by reproducing generation with the supplied Swagger 2.0 definition through the swagger-codegen-maven-plugin on version 2.3.1, then compare the 2.4.x behavior. Review related issue #9509 for the escaping context. Done means scalar references receive @Pattern and regexes such as \d+ produce compilable generated code.