[scala][sttp]convert header parameter to string

[bgong-mdsol]

This PR fixes the issue #11942

Here is the openapi spec for reproduce the issue and verify the fix.

openapi: 3.0.0
info:
  title: Request Header
  version: "1.0.0"
paths:
  /test:
    get:
      summary: Empty Route
      parameters:
      - description: Gets user info.
        explode: false
        in: header
        name: user_uuid
        required: true
        schema:
          format: uuid
          type: string
        style: simple
      responses:
        "200":
          description: Good Request.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ReturnValue'
        "400":
          description: Bad Request.
components:
  schemas:
    ReturnValue:
      properties:
        name:
          type: string

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[wing328]

LGTM