Fix for not closed GzipSource

[vbuberen]

📄 Context

There is an issue #472
Issue appears with StrictMode enabled. We couldn't catch the issue earlier, because sample app has no gzip request with body, so code with error wasn't reachable.

📝 Changes

• Fixed the issue by adding use { } function to GzipSource in getNativeSource() function.
• Updated tests to use real Buffer(), since Mockk can't properly react to close() call.
• Added a fake Gzip request which I fill with some dummy request body and header telling that response is with gzip encoding. It returns 405, but allows to reproduce the issue.
• Added StrictMode to the sample app, because this is not the first time we have issues reported by people with this mode enabled (for example StrictMode policy violation: android.os.strictmode.LeakedClosableViolation #421)
• (UPD: after review) Renamed existing gzip request to explicitly mention that it returns just gzip response.

🛠️ How to test

To reproduce the issue remove use from getNativeSource() and use just GzipSource directly. With StrictMode enabled sample will crash when fakeGzipRequest() is called.

⏱️ Next steps

We might want to improve the part of request processing and cover it with some more tests.
Closes #472

[MiSikora]

Left one minor thing.

As far as the requests revamp go I think we should go with a similar route to responses processing.