fix primitive type of param in body; use const string for type

[sdghchj]

Fix a bug by last PR: wrong primitive type of param in body;
Use const string for type.

[codecov-commenter]

Codecov Report

Merging #734 into master will decrease coverage by 0.09%.
The diff coverage is 86.15%.

@@            Coverage Diff             @@
##           master     #734      +/-   ##
==========================================
- Coverage   84.58%   84.48%   -0.10%     
==========================================
  Files           7        7              
  Lines        1790     1792       +2     
==========================================
  Hits         1514     1514              
- Misses        171      173       +2     
  Partials      105      105              

Impacted Files	Coverage Δ
operation.go	87.56% <76.19%> (-0.32%)	⬇️
parser.go	79.76% <88.46%> (ø)
property.go	91.66% <88.88%> (ø)
schema.go	96.82% <100.00%> (ø)

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[easonlin404]

LGTM