Issue/1600 fix element property

[mAiNiNfEcTiOn]

Thanks in advance for your contribution. Please follow the below steps in submitting a pull request, as it will help us with addressing it quicker.

• Create a new branch from master (e.g. features/my-new-feature or issue/123-my-bugfix)
• If you're fixing a bug also create an issue if one doesn't exist yet
• If it's a new feature explain why do you think it's necessary
• If your change include drastic or low level changes please discuss them to make sure they will be accepted and what the impact will be
• If your change is based on existing functionality please consider refactoring first. Pull requests that duplicate code will not make it in very quick, if at all.
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• Add unit tests

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• This PR contains @yak80's changes on PR Fix probs with latest gecko-driver(selenium/standalone-firefox:3.5.0 and above) #1604 and changes the mocks.json to make the tests actually pass.

• Also moves the validateElementEntry out of the postRequest function to not recreate it at every request.

Closes #1600

/cc @yak80

[beatfactor]

Thanks, but I'm already reviewing #1604.