Fix type inference error

Created by: ackintosh

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Error message:

Type inference failed. Expected type mismatch: inferred type is Map<String, Boolean> but Map<String, String> was expected

$ cd samples/openapi3/client/petstore/kotlin
$ ./gradlew check assemble
Starting a Gradle Daemon, 14 incompatible Daemons could not be reused, use --status for details
:compileKotlin
Using kotlin incremental compilation
e: /Users/akihito1/src/github.com/ackintosh/openapi-generator-1/samples/openapi3/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/apis/FakeApi.kt: (342, 89): Type inference failed. Expected type mismatch: inferred type is Map<String, Boolean> but Map<String, String> was expected