Avoid encoding list of spans twice in BaseHttpSender (#270)

newBody was called twice which would result in encoding the list of
spans twice unnecessarily. This change will reduce allocations and
copying memory.

core/src/main/java/zipkin2/reporter/BaseHttpSender.java

@@ -120,7 +120,7 @@ final U nextEndpoint(HttpEndpointSupplier endpointSupplier) {
  if (endpoint == null) endpoint = nextEndpoint(endpointSupplier);
  B body = newBody(encodedSpans);
  if (body == null) throw new NullPointerException("newBody(encodedSpans) returned null");
- postSpans(endpoint, newBody(encodedSpans));
+ postSpans(endpoint, body);
  }
 
  @Override public final void close() {