Fix kotlin client issue in maven plugin tests

• Fix kotlin client issue in maven plugin tests:

[ERROR] /Users/williamcheng/Code/openapi-generator/modules/openapi-generator-maven-plugin/examples/target/generated-sources/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt: (183, 81) Unresolved reference: lowercase
[ERROR] /Users/williamcheng/Code/openapi-generator/modules/openapi-generator-maven-plugin/examples/target/generated-sources/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt: (198, 74) Unresolved reference: lowercase
[ERROR] Failed to execute goal org.jetbrains.kotlin:kotlin-maven-plugin:1.5.10:compile (compile) on project sample-project: Compilation failure: Compilation failure: 
[ERROR] /Users/williamcheng/Code/openapi-generator/modules/openapi-generator-maven-plugin/examples/target/generated-sources/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt:[183,81] Unresolved reference: lowercase
[ERROR] /Users/williamcheng/Code/openapi-generator/modules/openapi-generator-maven-plugin/examples/target/generated-sources/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt:[198,74] Unresolved reference: lowercase

• Update doc

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