Fix value accumulation for case-sensitive keys in NettyHeadersAdapter

[qnnn]

The current NettyHeaders supports case-sensitive storage of entry values when using the add method. This causes the NettyHeadersAdapter to accumulate the values for the corresponding key when retrieving headers(see unit test).

Add a unit test testHeadersOutput to verify the final output of the headers.

[qnnn]

addAll has the same issue, and it has also been addressed.

[simonbasle]

hey @qnnn thanks for the PR. Can you elaborate on the use case that lead you to come up with this change? I'm guessing this is a matter of enumerating the headers or representing them as a whole?

[qnnn]

hey @qnnn thanks for the PR. Can you elaborate on the use case that lead you to come up with this change? I'm guessing this is a matter of enumerating the headers or representing them as a whole?

@simonbasle I discovered this problem while using the AddRequestHeader GatewayFilter in Spring Cloud Gateway. Through network packet capture, it can be seen that the HTTP requests sent by the Gateway aggregate the values of case-sensitive keys in the HTTP headers.
The expected request header should be testheader=first, second, but the actual result is testheader=first, second, first, second (the same as the output in the unit test).