fix getAttchments return copy map problem.

[horizonzy]

What is the purpose of the change

revert #8772.
add unit case.

[codecov-commenter]

Codecov Report

Merging #9571 (9c17431) into master (8240ea2) will decrease coverage by 0.77%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #9571      +/-   ##
============================================
- Coverage     60.89%   60.12%   -0.78%     
+ Complexity      448      444       -4     
============================================
  Files          1100     1099       -1     
  Lines         44520    44507      -13     
  Branches       6478     6475       -3     
============================================
- Hits          27111    26759     -352     
- Misses        14441    14786     +345     
+ Partials       2968     2962       -6     

Impacted Files	Coverage Δ
...rc/main/java/org/apache/dubbo/rpc/AppResponse.java	56.96% <100.00%> (+1.26%)	⬆️
.../java/org/apache/dubbo/rpc/AttachmentsAdapter.java	59.09% <100.00%> (+59.09%)	⬆️
...src/main/java/org/apache/dubbo/rpc/RpcContext.java	66.83% <100.00%> (+0.49%)	⬆️
.../main/java/org/apache/dubbo/rpc/RpcInvocation.java	64.73% <100.00%> (ø)
...ookeeper/ZookeeperDynamicConfigurationFactory.java	0.00% <0.00%> (-66.67%)	⬇️
...gcenter/wrapper/CompositeDynamicConfiguration.java	0.00% <0.00%> (-63.34%)	⬇️
...fig/configcenter/TreePathDynamicConfiguration.java	25.71% <0.00%> (-60.01%)	⬇️
...pport/zookeeper/ZookeeperDynamicConfiguration.java	0.00% <0.00%> (-60.00%)	⬇️
.../dubbo/metadata/report/MetadataReportInstance.java	12.50% <0.00%> (-40.63%)	⬇️
.../configcenter/support/zookeeper/CacheListener.java	0.00% <0.00%> (-32.44%)	⬇️
... and 56 more

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