is (type, ...types) {
    return typeis(this.req, type, ...types)
  }
Check if the incoming request contains the "Content-Type"
header field and if it contains any of the given mime `type`s.
If there is no request body, `null` is returned.
If there is no content type, `false` is returned.
Otherwise, it returns the first `type` that matches.

Examples:

    // With Content-Type: text/html; charset=utf-8
    this.is('html'); // => 'html'
    this.is('text/html'); // => 'text/html'
    this.is('text/*', 'application/json'); // => 'text/html'

    // When Content-Type is application/json
    this.is('json', 'urlencoded'); // => 'json'
    this.is('application/json'); // => 'application/json'
    this.is('html', 'application/*'); // => 'application/json'

    this.is('html'); // => false

@param {String|String[]} [type]
@param {String[]} [types]
@return {String|false|null}
@api public