Remove required fields from evalResult swagger model

[zhouzhuojie]

Description

This caused an issue under the following circumstance:

• A flag did not exist in flagr
• The consumer requested a flag by only specifying :flag_key
• Flagr returned a 200 response with :flag_id omitted
• The ruby code generated by swagger raised an ArgumentError because the response from the server was missing :flag_id

Ultimately, it's up to the consumer app anyway to decide what it requires from the server response, not the HTTP interface library.

Motivation and Context

How Has This Been Tested?

Unit test and integration tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #239 into master will decrease coverage by 1.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #239      +/-   ##
==========================================
- Coverage   83.69%   82.62%   -1.07%     
==========================================
  Files          25       25              
  Lines        1441     1439       -2     
==========================================
- Hits         1206     1189      -17     
- Misses        177      192      +15     
  Partials       58       58

Impacted Files	Coverage Δ
pkg/handler/data_recorder_kinesis.go	81.57% <ø> (ø)	⬆️
pkg/handler/eval.go	77.01% <100%> (-0.27%)	⬇️
pkg/config/middleware.go	64.34% <0%> (-11.63%)	⬇️

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[zhouzhuojie]

Since go 1.12, monkey.PatchInstanceMethod is not working anymore, thus remove this hacky monkey patch

[zhouzhuojie]

Tested the change with rbflagr 0.1.2 (which still keeps the required fields of evalResult), looks good for the SDKs

[erithmetic]

This looks good, we verified that if only flag_key is requested and the flag does not exist, flagr will respond with flag_id removed from the response as the serializer automatically removes items with default values.