has (field) {
    return typeof this.res.hasHeader === 'function'
      ? this.res.hasHeader(field)
      // Node < 7.7
      : field.toLowerCase() in this.headers
  }
Returns true if the header identified by name is currently set in the outgoing headers.
The header name matching is case-insensitive.

Examples:

    this.has('Content-Type');
    // => true

    this.get('content-type');
    // => true

@param {String} field
@return {boolean}
@api public