fix alibaba RpcContext setAttachment(String,String)

[chickenlj]

com.alibaba.dubbo.rpc.RpcContext getAttachment(String, String) cannot work properly for some other middlewares integrating with Dubbo.

[codecov-io]

Codecov Report

Merging #6052 into master will decrease coverage by 0.39%.
The diff coverage is 3.77%.

@@             Coverage Diff              @@
##             master    #6052      +/-   ##
============================================
- Coverage     61.25%   60.85%   -0.40%     
+ Complexity      494      490       -4     
============================================
  Files           991      991              
  Lines         39582    39636      +54     
  Branches       5716     5690      -26     
============================================
- Hits          24246    24121     -125     
- Misses        12663    12834     +171     
- Partials       2673     2681       +8     

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/com/alibaba/dubbo/rpc/RpcContext.java	7.07% <3.77%> (-85.23%)	0.00 <0.00> (ø)
...va/org/apache/dubbo/remoting/TimeoutException.java	0.00% <0.00%> (-33.34%)	0.00% <0.00%> (ø%)
...rg/apache/dubbo/remoting/utils/PayloadDropper.java	30.76% <0.00%> (-15.39%)	0.00% <0.00%> (ø%)
...ng/transport/dispatcher/WrappedChannelHandler.java	47.82% <0.00%> (-15.22%)	0.00% <0.00%> (ø%)
...dubbo/remoting/exchange/support/DefaultFuture.java	75.00% <0.00%> (-13.80%)	0.00% <0.00%> (ø%)
...ache/dubbo/remoting/transport/AbstractChannel.java	75.00% <0.00%> (-12.50%)	0.00% <0.00%> (ø%)
...che/dubbo/remoting/transport/mina/MinaChannel.java	39.47% <0.00%> (-10.53%)	15.00% <0.00%> (-1.00%)
.../apache/dubbo/rpc/protocol/AsyncToSyncInvoker.java	72.41% <0.00%> (-10.35%)	0.00% <0.00%> (ø%)
...ng/transport/dispatcher/all/AllChannelHandler.java	62.06% <0.00%> (-10.35%)	0.00% <0.00%> (ø%)
.../dubbo/remoting/transport/netty4/NettyChannel.java	60.39% <0.00%> (-7.93%)	0.00% <0.00%> (ø%)
... and 39 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dbda449...292f861. Read the comment docs.

[hengyunabc]

LGTM

[qinliujie]

LGTM