Open API spec should include path fragment specified as --api-path parameter

[jfbourgon]

When starting Franklin with --api-path=mypath, the content of the Open api spec document doesn't reflect that the API is available as a deeper location then the web server root. This creates invalid links when you Try them out in a swagger frontend.

Snippet of the current content of the open-api/spec.yaml (ignoring --api-path)

openapi: 3.0.3
info:
  title: Franklin
  version: 0.0.1
paths:
  /collections:
    get:
      description: A list of collections
      operationId: collections
      responses:
        '200':
          description: ''
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Json'
  /collections/{p1}:
    get:
      description: A single collection
      operationId: collectionUnique
      parameters:
      - name: p1
        in: path
        required: true
        schema:
          type: string
      responses:
        '200':
          description: ''
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Json'
        '404':
          description: not found
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/NotFound'

Recommended content of the open-api/spec.yaml when --api-path is set

openapi: 3.0.3
info:
  title: Franklin
  version: 0.0.1
paths:
  /mypath/collections:
    get:
      description: A list of collections
      operationId: collections
      responses:
        '200':
          description: ''
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Json'
  /mypath/collections/{p1}:
    get:
      description: A single collection
      operationId: collectionUnique
      parameters:
      - name: p1
        in: path
        required: true
        schema:
          type: string
      responses:
        '200':
          description: ''
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Json'
        '404':
          description: not found
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/NotFound'
      ...