fix(proto): fix getting attributes issue

[justxuewei]

RPCInvocation::GetAttributeWithDefaultValue() has a typo which couldn't
get attributes correctly, this PR fixes that issue.

Signed-off-by: Xuewei Niu justxuewei@apache.org

What this PR does:

Which issue(s) this PR fixes:
Fixes #

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• All ut passed (run 'go test ./...' in project root)
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• Your new-created file needs to have apache license at the top, like other existed file does.
• All integration test passed. You can run integration test locally (with docker env). Clone our dubbo-go-samples project and replace the go.mod to your dubbo-go, and run 'sudo sh start_integration_test.sh' at root of samples project root. (M1 Slice is not Support)

[codecov-commenter]

Codecov Report

Merging #1968 (0d7edc2) into master (f164c81) will decrease coverage by 0.07%.
The diff coverage is 30.76%.

@@            Coverage Diff             @@
##           master    #1968      +/-   ##
==========================================
- Coverage   44.48%   44.41%   -0.08%     
==========================================
  Files         284      283       -1     
  Lines       17038    17023      -15     
==========================================
- Hits         7580     7560      -20     
- Misses       8654     8656       +2     
- Partials      804      807       +3     

Impacted Files	Coverage Δ
cluster/cluster/base/cluster_invoker.go	24.44% <ø> (ø)
cluster/cluster/broadcast/cluster_invoker.go	80.00% <ø> (ø)
cluster/cluster/failback/cluster_invoker.go	75.82% <ø> (ø)
cluster/cluster/failover/cluster_invoker.go	66.66% <ø> (ø)
cluster/cluster/failsafe/cluster_invoker.go	81.81% <ø> (ø)
cluster/cluster/forking/cluster_invoker.go	58.33% <ø> (ø)
cluster/loadbalance/p2c/loadbalance.go	76.36% <ø> (ø)
cluster/loadbalance/ringhash/ringhash.go	63.33% <ø> (ø)
cluster/router/tag/match.go	90.69% <ø> (ø)
cluster/router/tag/router.go	59.64% <ø> (ø)
... and 128 more

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