[PHP] Allow strings with only whitespaces in enums

Created by: thomasphansen

The current implementation of the PHP client doesn't allow strings containing only white spaces (like ' ') as values for enum, despite they being valid in OpenAPI specification (or, at least not forbidden). This is a fragment of Microsoft Business Central API for reading/posting a Journal Entry (I'm only showing the relevant field):

components:
  schemas:
    JournalEntry:
      - type: object
        properties:
          'Document_Type':
            type: string
            enum:
              - ' '
              - 'Payment'
              - 'Invoice'
              - 'Credit Memo'
              - 'Finance Charge Memo'
              - 'Reminder'
              - 'Refund'
            default: ' '

Microsoft Business Central API demands the default value to be a string with exactly one space character inside it, otherwise it fails. The current PHP generator sanitizes the enum options, removing all white spaces - even if the option contain only white spaces. This PR changes the generator, so that it accepts whitespace-only strings as valid enum options and name them as SPACE_X, where X is the number of spaces inside that string. The change does NOT affect the trimming of enum options containing characters other than spaces - which means, this should not be a BC: it only affect a currently unsupported scenario.

We already use this change in our own branch of openapi-generator - I'm creating some PRs upstream, so that we can share the small bugs we have fixed, and this is one of them! 😄

The immediate benefit of this PR is to allow the OpenAPIGenerator PHP client to work with Microsoft Business Central, but it will surely benefit other APIs using space-only strings as enum values. 😉

Cheers!

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