Context leak for streaming client requests

[llinder]

The HTTP client wrapper exhibits a context memory leak when using streaming requests. The effected code is likely here https://github.com/openzipkin-contrib/brave-ratpack/blob/master/src/main/java/ratpack/zipkin/internal/ZipkinHttpClientImpl.java#L129

This issue was originally reported by @Bijnagte and also reported that the memory leak went away if code was switched back to use a non streaming request.