[Kotlin-spring] fix default value for @ApiParam

Created by: frecco75

This PR fixes default value for String into @ApiParam.

For example given this schema :

openapi: "3.0.1"
info:
  version: 1.0.0
  title: Users

paths:
  /users/search:
    post:
      parameters:
        - name: name
          in: header
          description: The name of the user
          schema:
            type: string
            default: "bond"
      responses:
        default:
          description: ""

The generator without the fix produces this code which doesn't compile because of invalid defaultValue attribute

    fun usersSearchPost(@ApiParam(value = "The name of the user" , defaultValue=""bond"") @RequestHeader(value="name", required=false) name: kotlin.String
): ResponseEntity<Unit>

with the fix it compiles correctly

fun usersSearchPost(@ApiParam(value = "The name of the user" , defaultValue="bond") @RequestHeader(value="name", required=false) name: kotlin.String
): ResponseEntity<Unit>