use AppendHeader for http2

[lhsoft]

What problem does this PR solve?

Issue Number:

Problem Summary:

What is changed and the side effects?

Changed:

Side effects:

• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

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Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
• Please follow Contributor Covenant Code of Conduct.(请遵循贡献者准则).

[wwbmmm]

The original test case was written as "overwrite-a", was it expected to be overwritten? Changing the behavior here may not conform to the previous design.

[lhsoft]

The test case is http2_header_after_data.
For http2, according to the rfc , it shouldn't be expected to be overwritten.
The grpc use map[string][]string for header:

md := metadata.Pairs(
    "key1", "val1",
    "key1", "val1-2", // "key1" will have map value []string{"val1", "val1-2"}
    "key2", "val2",
)

[wwbmmm]

Do you know why it was designed to overwrite before? @zyearn

[zyearn]

RFC doesn't mention overwritten. It says: Multiple message-header fields with the same field-name MAY be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)].

In brpc implementation, if the incoming header is in the format of "key:value1,value2,value3", brpc won't overwrite cases like this. But combining headers seems reasonable otherwise the previous header would be lost.

[zyearn]

LGTM