EdgeAddHeaderClientFilter只能传递单个值的header

## 问题版本

Java-Chassis 2.8.x

## 关键代码

详见:
https://github.com/apache/servicecomb-java-chassis/blob/5f327e6bfc12cf522649c57d156296b0670609c6/edge/edge-core/src/main/java/org/apache/servicecomb/edge/core/EdgeAddHeaderClientFilter.java#L91-L97

`oldRequest.getHeader(key)` 只能取出header的单个值, 如果`key`代表的header有多个值, 则其余header值都不会被透传.

Start in edge/edge-core/src/main/java/org/apache/servicecomb/edge/core/EdgeAddHeaderClientFilter.java at lines 91-97 and inspect how oldRequest.getHeader(key) handles repeated values. Reproduce a request with a multi-valued header and verify that completion means every value is forwarded rather than only one; the payload names no test, so locate relevant edge-core coverage before running it.

Written by the indexing model from the issue text.