Ftr/support register with no pojo object

[LaurenceLiZhixin]

What this PR does:
Add not pojo object register support, to support dubbo-go-cli tool.
Before, we can't register an object that made by "reflect.New" because it's not impl pojo interface.
But now, we can register this object, as long as it contains field "JavaClassName"

Which issue(s) this PR fixes:

NONE
Special notes for your reviewer:
Create a function to deal with interface made by reflect, get JavaClassName field and register the object with javaClassName.

Does this PR introduce a user-facing change?:
NONE

[wongoo]

@LaurenceLiZhixin I think the func can meet your requirement ： func RegisterPOJOMapping(javaClassName string, o interface{}) int

[LaurenceLiZhixin]

@LaurenceLiZhixin I think the func can meet your requirement ： func RegisterPOJOMapping(javaClassName string, o interface{}) int

I think this function can register object, and I actually use this funciton in my code.
But if I want to encode a none POJO obj, hessian2 can't find out what the object is, because it can't identify the object without JavaClassName. That is what my codes do: find the JavaClassName field and use it to encode registered object.

[wongoo]

@LaurenceLiZhixin thanks for submit this pr.

1. it's not efficient to check whether it's exist a field named JavaClassName, u should call checkPOJORegistry first to check whether it's being registered or not.
2. SHOULD NOT copy a whole method and just change a little logic in it, a better way is to create a new func encNamedPOJOObject(javaClassName string, v interface{}) , and the original func encPOJOObject calling the new func.

[codecov-io]

Codecov Report

Merging #243 (acceb0a) into master (d93e48c) will decrease coverage by 0.14%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #243      +/-   ##
==========================================
- Coverage   66.24%   66.10%   -0.15%     
==========================================
  Files          25       25              
  Lines        2776     2785       +9     
==========================================
+ Hits         1839     1841       +2     
- Misses        711      716       +5     
- Partials      226      228       +2     

Impacted Files	Coverage Δ
encode.go	84.04% <0.00%> (ø)
object.go	64.39% <36.36%> (-1.21%)	⬇️

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[wongoo]

LGTM