Fix for #284 - Match platform without considering case

[vikmaksymenko]

It looks like Selenium 4 and Selenium 3 submit "platform" capability differently:

• for the 3rd version it's upper-case (e.g. "chrome-107.0-LINUX")
• for the 4th version it's lower-case (e.g. "chrome-107.0-linux")

We want our GGR server to work with both Selenium versions, so I used strings.ToLower for platform filtering to become case insensitive.

[codecov[bot]]

Codecov Report

Base: 87.91% // Head: 87.91% // No change to project coverage :thumbsup:

Coverage data is based on head (f239b9e) compared to base (980de9f). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #372 +/- ## ======================================= Coverage 87.91% 87.91% ======================================= Files 4 4 Lines 596 596 ======================================= Hits 524 524 Misses 54 54 Partials 18 18 ``` | Flag | Coverage Δ | | |---|---|---| | go | `87.91% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aerokube#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/aerokube/ggr/pull/372?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aerokube) | Coverage Δ | | |---|---|---| | [config.go](https://codecov.io/gh/aerokube/ggr/pull/372?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aerokube#diff-Y29uZmlnLmdv) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aerokube). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aerokube)

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[vikmaksymenko]

Let's add a test similar to this but with different case in config and capabilities: https://github.com/aerokube/ggr/blob/master/config_test.go#L154-L159

Done in f239b9ee1b32963c6854f858bbbd19ac5ef42770