Donghyeon Kim

@velo I pushed some changes 1. Refactor some code - Use `FutureKt.await` instead of [KotlinExtensions.awaitResponse](https://github.com/OpenFeign/feign/blob/80f960e32c4ce08760ecb52e9ae8f9c4499f15cc/core/src/main/java/feign/KotlinExtensions.kt#L12-L13) - Renamed KotlinExtensions to MethodKt 2. Update the Kotlin version - If the Feign library...

@velo ~I'm looking into other libraries to isolate the Kotlin support module so that the core module doesn't reference the Kotlin library, but Spring is using Kotlin in the core...

@velo > But, until this is moved to a module, I wouldn't be comfortable to merge, as core needs to be lean and have no dependencies. I'm considering splitting modules...

@velo I pushed some changes to share idea. - Because the optional dependency is used, Kotlin support related libraries will not be referenced in a Java-only project. 407ab47 - Assert.java...

@velo Looking at the final result, `feign-core` depends on `feign-kotlin` as an optional dependency. I think it's a bit better than relying on kotlin-related dependencies directly, but as you said,...

@velo Looking at the Github Example project, it seems that feign-core is used with zero dependency. Am I misunderstanding something?

@velo > either that dependency tree wrong, or a runtime exception will happen when performing an async operation I checked that Github Example in Feign project runs correctly without kotlin...

Thanks for the additional work. I've been too busy lately. I'll be able to start working again in two weeks. I'll finish this PR soon.

Branch updated to latest state ```shell $ git rebase master && git push --force ```

1. This Branch updated to latest master branch ```shell $ git rebase master && git push --force ``` 2. Add a single commit [78589d5](https://github.com/OpenFeign/feign/pull/1706/commits/78589d5f4f9fa93e1388b6c90a0b03aefcc11501)