removed field 'type': 'object' from swagger json when using doc.Object()

[AHBaber]

This is a new pull request based around the same issue as in pull/145

This version has two differences:

1. it implements the smaller change has discussed in the original request
2. the master branch was updated to contain the commits made after the original pull request was initially made.

[codecov-io]

Codecov Report

Merging #149 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #149   +/-   ##
=======================================
  Coverage   95.22%   95.22%           
=======================================
  Files           5        5           
  Lines         461      461           
  Branches       94       94           
=======================================
  Hits          439      439           
  Misses          7        7           
  Partials       15       15

Impacted Files	Coverage Δ
sanic_openapi/doc.py	99.08% <ø> (ø)	⬆️

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[chenjr0719]

LGTM