Fix map decode error issue #259

[wongoo]

What this PR does:
Fix map decode error issue

Which issue(s) this PR fixes:

Fixes #259

Special notes for your reviewer:
the map type should be added into type map, not the class def map. see http://hessian.caucho.com/doc/hessian-serialization.html##type-map

Does this PR introduce a user-facing change?:

- Fix map decode error issue #259 

[codecov-io]

Codecov Report

Merging #261 (9696ab9) into master (db6d4e2) will decrease coverage by 0.45%.
The diff coverage is 51.02%.

@@            Coverage Diff             @@
##           master     #261      +/-   ##
==========================================
- Coverage   67.41%   66.96%   -0.46%     
==========================================
  Files          25       25              
  Lines        2655     2658       +3     
==========================================
- Hits         1790     1780      -10     
- Misses        644      657      +13     
  Partials      221      221              

Impacted Files	Coverage Δ
map.go	52.77% <42.30%> (-2.10%)	⬇️
decode.go	69.09% <60.86%> (-2.91%)	⬇️
pojo.go	59.65% <0.00%> (-5.69%)	⬇️
double.go	87.14% <0.00%> (+1.65%)	⬆️

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