Support brotli encoding in HAR capture #739

When capturing responses for a HAR, response content that is brotli-compressed (identified by a Content-Encoding header with value "br") is not decompressed. This results in the HAR containing base64-encoded content for those responses where it would be helpful to have plain text. Scripts and stylesheets are frequently served with brotli compression.

Currently, the ServerResponseCaptureFilter class only supports decompressing gzip-compressed response content in the decompressContents() method:

protected void decompressContents() {
    if (contentEncoding.equals(HttpHeaders.Values.GZIP)) {
        try {
            fullResponseContents = BrowserMobHttpUtil.decompressContents(getRawResponseContents());
            decompressionSuccessful = true;
        } catch (RuntimeException e) {
            log.warn("Failed to decompress response with encoding type " + contentEncoding + " when decoding request from " + originalRequest.getUri(), e);
        }
    } else {
        log.warn("Cannot decode unsupported content encoding type {}", contentEncoding);
    }
}

It would be helpful to support decompression of brotli-compressed content in this class, and moreover, it would be helpful to support creation of a custom ServerResponseCaptureFilter in the HarCaptureFilter constructor, perhaps by providing a factory as a constructor parameter.

There are some other technically valid content encodings that are not supported by the current server response filter; for example, the Content-Encoding header might be "identity" or might specify multiple encodings. The factory approach would allow Browsermob users to implement their own response content manipulations as needed.

The library org.brotli:dec:0.1.1 provides a pure-Java brotli decompression implementation that could be used for this purpose. I'd be glad to try out a patch to support brotli and submit a pull request for the functionality suggested here.

(Tested with version 2.1.5.)