Do not send null values in cookie

[JorisVanEijden]

Both JsonWireProtocol and w3c protocol say to leave an entry off
rather than having a null value.

https://github.com/SeleniumHQ/selenium/wiki/JsonWireProtocol
https://w3c.github.io/webdriver/#add-cookie

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[JorisVanEijden]

The travis fail seems to be unrelated to this PR:

1) Facebook\WebDriver\WebDriverTimeoutsTest::testShouldFailGettingDelayedElementWithoutWait
Failed asserting that exception of type "Facebook\WebDriver\Exception\NoSuchElementException" is 
thrown.

How does this happen? And what can I do to fix it?

The code coverage decreased even though every single line I touched is tested and I added an extra test. How does this happen? And what can I do to fix it?

[derekmd]

With Chromedriver 74 binaries released, it now causes some cookie requests to fail without this pull request being merged and tagged.

https://chromium-review.googlesource.com/c/chromium/src/+/1366925/3/chrome/test/chromedriver/window_commands.cc#1564

As a workaround, you must manually set every cookie value to avoid null Set-Cookie items being sent.

[OndraM]

Thanks @JorisVanEijden for the catch. I rebased the branch against current community branch, let's see if the Travis build now pass. If so, I will get it merged.

[OndraM]

Merged, thanks @JorisVanEijden! 💯