Motivation:
We currently append extensions to the user defined "sec-websocket-extensions" headers. This can cause duplicated entries.
Modifications:
* Replace existing `WebSocketExtensionUtil#appendExtension` private helper with a new `computeMergeExtensionsHeaderValue`. User defined parameters have higher precedence.
* Add tests (existing method wasn't tested)
* Reuse code for both client and server side (code was duplicated).
Result:
No more duplicated entries when user defined extensions overlap with the ones Netty generated.
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