Fix hessian codec bugs

[zonghaishang]

What this PR does:

Which issue(s) this PR fixes:

Fixes #164
Fixes #168

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[zonghaishang]

@wongoo Please help check TestList test case, local run is passed

https://travis-ci.org/github/apache/dubbo-go-hessian2/builds/663769046?utm_source=github_status&utm_medium=notification

error stack:

=== RUN   TestList
--- FAIL: TestList (0.00s)
panic: assignment to entry in nil map [recovered]
	panic: assignment to entry in nil map
goroutine 484 [running]:
testing.tRunner.func1(0xc00039c500)
	/home/travis/.gimme/versions/go1.11.1.linux.amd64/src/testing/testing.go:792 +0x6a7
panic(0x88d280, 0x95fe00)
	/home/travis/.gimme/versions/go1.11.1.linux.amd64/src/runtime/panic.go:513 +0x1b9
github.com/apache/dubbo-go-hessian2.RegisterPOJO(0x960880, 0xc10c20, 0x0)
	/home/travis/build/apache/dubbo-go-hessian2/pojo.go:149 +0x2f9
github.com/apache/dubbo-go-hessian2.RegisterPOJOs(0xc0001b9f38, 0x3, 0x3, 0xc0000a4cc0, 0x871c00, 0x95e650)
	/home/travis/build/apache/dubbo-go-hessian2/pojo.go:218 +0xf6
github.com/apache/dubbo-go-hessian2.TestList(0xc00039c500)
	/home/travis/build/apache/dubbo-go-hessian2/list_test.go:75 +0x125
testing.tRunner(0xc00039c500, 0x906fc8)
	/home/travis/.gimme/versions/go1.11.1.linux.amd64/src/testing/testing.go:827 +0x163
created by testing.(*T).Run
	/home/travis/.gimme/versions/go1.11.1.linux.amd64/src/testing/testing.go:878 +0x651
exit status 2
FAIL	github.com/apache/dubbo-go-hessian2	30.313s

[codecov-io]

Codecov Report

Merging #167 into master will increase coverage by 0.96%.
The diff coverage is 87.87%.

@@            Coverage Diff             @@
##           master     #167      +/-   ##
==========================================
+ Coverage   66.45%   67.42%   +0.96%     
==========================================
  Files          21       22       +1     
  Lines        2573     2671      +98     
==========================================
+ Hits         1710     1801      +91     
+ Misses        667      666       -1     
- Partials      196      204       +8

Impacted Files	Coverage Δ
response.go	56.37% <100%> (+7.11%)	⬆️
object.go	64.61% <66.66%> (+1%)	⬆️
pojo.go	60.1% <91.66%> (+3.56%)	⬆️
java_collection.go	54.66% <0%> (ø)
list.go	85.38% <0%> (+0.27%)	⬆️
codec.go	65.83% <0%> (+0.83%)	⬆️
hessian.go	48.14% <0%> (+8.33%)	⬆️

---

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[fangyincheng]

LGTM

[wongoo]

LGTM