OkHttpClient.SetAuthenticator missing

[Morten Nielsen]

The OkHttpClient class has an "Authenticator" read-only property, but is missing the SetAuthenticator method. This prevents me from porting the NTLM support demonstrated here: square/okhttp#206 (comment)

https://github.com/square/okhttp/blob/a87f4b4585fe6e8e49c7760986fd5d2c591e0573/okhttp/src/main/kotlin/okhttp3/OkHttpClient.kt#L632-L640