Unify `browserName` given through different sources

[aik099]

The browser name can be specified through these places:

• the $browserName driver constructor argument;
• the $desiredCapabilities['browserName'] key of the desired capabilities (either given upon driver construction or later on).

Both browser names have no relation to each other which can lead to inconsistent driver configuration, e.g. $this->browserName set to the chrome, but in $this->desiredCapabilties['browserName'] set to the firefox.

P.S. Nobody was noticing any bugs, because the \WebDriver\WebDriver::session method (a dependency of this project) has a polyfill code, that does the same thing as this PR.

The above-mentioned polyfill code is removed in 2.x releases of the WebDriver, which allowed us to spot this problem.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.21%. Comparing base (a3a5370) to head (a6cfcfa).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #390 +/- ## ============================================ + Coverage 90.19% 90.21% +0.02% Complexity 168 168 ============================================ Files 1 1 Lines 469 470 +1 ============================================ + Hits 423 424 +1 Misses 46 46 ```

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[aik099]

Thank you, @stof .