close the APM client response body regardless of the response

[ivantopo]

On 2.5.10 we moved to use the APM v2 ingestion API which doesn't return a response body, but OkHttp requires response bodies to be closed regardless of whether there is a body. Without this fix, applications end up logging this warning:

WARNING: A connection to https://ingestion.apm.kamon.io/ was leaked. Did you forget to close a response body? To see where this was allocated, set the OkHttpClient logger level to FINE: Logger.getLogger(OkHttpClient.class.getName()).setLevel(Level.FINE);