fix parse basic type bug

[luky116]

When accessing a method whose return value is an enume value on the server, the client cannot parse the return value and reports an error. The tracing code found that the client could not convert the enum value type to Int64 type

You can reproduce the above problem through this code：https://github.com/luky116/dubbo-go-samples/tree/fix-rpcDubbo/rpc/dubbo

[codecov-commenter]

Codecov Report

Merging #298 (d26d6da) into master (75ff950) will increase coverage by 0.35%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #298      +/-   ##
==========================================
+ Coverage   70.35%   70.70%   +0.35%     
==========================================
  Files          27       27              
  Lines        3049     3021      -28     
==========================================
- Hits         2145     2136       -9     
+ Misses        679      660      -19     
  Partials      225      225              

Impacted Files	Coverage Δ
codec.go	75.56% <33.33%> (+4.88%)	⬆️

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[wongoo]

LGTM