bugfix aligning service names

There is a difference in the substraction of service names in between implementations.  If a service "a" gets called without span information in the headers, the brave pre processor would name the service "/a" because of the javadocs: http://docs.oracle.com/javaee/6/api/javax/servlet/http/HttpServletRequest.html#getContextPath().  The client as a counterpart takes a subpart https://github.com/kristofa/brave/blob/master/brave-client/src/main/java/com/github/kristofa/brave/client/ClientRequestInterceptor.java getServiceName