Fix Session::{execute, execute_async}, must use value result

[mvorisek]

Fix release, 8bb204d refactoring has broken the Session::{execute, execute_async} methods.

parent::execute($jsonScript) is equivalent to:
$this->curl('POST', '/execute', $jsonScript)['value']
not
$this->curl('POST', '/execute', $jsonScript)

tested /w #110 (comment) example, ideally such example should be added to tests

[mvorisek]

pure cleanup as explicitly defined methods seems to be not defined in this list

[mvorisek]

pure cleanup, args are always required, otherwise Selenium throws:

  WebDriver\Exception\UnknownError: invalid argument: 'args' must be a list
        (Session info: headless chrome=84.0.4147.105)
      Build info: version: '3.141.59', revision: 'e82be7d358', time: '2018-11-14T08:25:53'
      System info: host: 'xxx', ip: 'xxx', os.name: 'Windows 10', os.arch: 'amd64', os.version: '10.0', java.version: '12.0.2'
      Driver info: driver.version: unknown in C:\...\vendor\instaclick\php-webdriver\lib\WebDriver\Exception.php:156
      Stack trace:
      #0 C:\...\vendor\instaclick\php-webdriver\lib\WebDriver\AbstractWebDriver.php(170): WebDriver\Exception::factory()
      #1 C:\...\vendor\instaclick\php-webdriver\lib\WebDriver\AbstractWebDriver.php(243): WebDriver\AbstractWebDriver->curl()
      #2 C:\...\vendor\instaclick\php-webdriver\lib\WebDriver\Container.php(241): WebDriver\AbstractWebDriver->__call()
      #3 C:\...\vendor\instaclick\php-webdriver\lib\WebDriver\Session.php(440): WebDriver\Container->__call()
      #4 src\Behat\Context.php(297): WebDriver\Session->execute()

[robocoder]

Thanks again.