Fix missing path parameters when also adding headers

[matgoebl]

mitmproxy2swagger adds path parameters like /{id} correctly as

      parameters:
      - name: id
        in: path
        required: true
        schema:
          type: string

When using mitmproxy2swagger --headers the headers are added, but the parameters will be missing:

      parameters:
      - name: content-type
        in: header
        required: false
        schema:
          type: string

The correct result should be

      parameters:
      - name: content-type
        in: header
        required: false
        schema:
          type: string
      - name: id
        in: path
        required: true
        schema:
          type: string

The reason for this bug is that if args.headers is used, the key parameters in swagger is already set and the later set_key_if_not_exists for params does not extend the key parameters.
One (least invasive) fix is to merge both arrays.