Pass options.headers send from the client to knox-mpu

I think this can also solve this (closed) https://github.com/balderdashy/skipper-s3/issues/1
This way when a user sends headers like this:
      bucket: process.env.S3_BUCKET_NAME,
      key: process.env.AWS_ACCESS_KEY_ID,
      secret: process.env.AWS_SECRET_ACCESS_KEY,
      headers: {
	    'Content-Type': 'image/jpeg',
	  }

The desired content type is set in S3