set type (type) {
    type = getType(type)
    if (type) {
      this.set('Content-Type', type)
    } else {
      this.remove('Content-Type')
    }
  }
Set Content-Type response header with `type` through `mime.lookup()`
when it does not contain a charset.

Examples:

    this.type = '.html';
    this.type = 'html';
    this.type = 'json';
    this.type = 'application/json';
    this.type = 'png';

@param {String} type
@api public