Project that illustrate how Kotlin simplifyies API tests
PRESENTATION
Kotlin idioms
Kotlin for grumpy developers
kotlin from beginning
Kotlin classes and objects (ru)
Habr about kotlin 1.1
Petstore project for test
roboPOJOGenerator plugin
assertjGen gradle plugin
Perhaps the best to study courses:
Introduction to Kotlin «O`Really»
Advanced Kotlin «O`Really»
Kotlin for Begginers «Udemy»