Content-Encoding: gzip header is not present when response filter is added

[amjd]

Describe the bug
When you add a custom response filter using any of the methods (BrowserUpProxy#addResponseFilter, BrowserUpProxy#addFirstHttpFilterFactory, BrowserUpProxy#addLastHttpFilterFactory), Content-Encoding: gzip header does not appear in the HAR, despite the server sending it. In addition, Content-Length header also seems to get updated with the uncompressed size of the response. So it seems that the proxy decompresses the gzip response, and also removes the evidence of compression in the process.

Interestingly, this only seems to occur with gzip-compressed responses and not brotli. I believe that may be because BUP / BMP doesn't support brotli decompression yet.

To Reproduce
Steps to reproduce the behavior:

1. Add Proxy with these settings, in this manner:

   // create and start proxy
   BrowserUpProxy proxy = new BrowserUpProxyServer();
   proxy.setTrustAllServers(true);
   proxy.start();
   
   // add response filter
   proxy.addResponseFilter((response, contents, info) -> System.out.println(info.getUrl() + " " + response.headers()));
   
   // start capturing HAR
   proxy.enableHarCaptureTypes(REQUEST_HEADERS, RESPONSE_HEADERS);
   proxy.newHar();

2. Open any website that supports gzip compression (e.g., https://www.example.com - real site) through the proxy, with "Accept-Encoding: gzip" header added to the request. I used the proxy with Selenium and Chrome browser.
3. Save HAR and check the response headers. Content-Encoding header would be missing from the response. Similar request through curl or browser includes the header.

   // save and analyze the HAR
   Har har = proxy.getHar();

Expected behavior
When a site supports gzip compression and Accept-Encoding header is sent with gzip value, server responds with Content-Encoding: gzip header and Content-Length header contains the compressed size. This information should be present in HAR as it was actually received.

Please complete the following information:

• OS: macOS, Linux
• Browser: Chrome
• Version: 85