Fix AutodiscoverService.tryGetEnabledEndpointsForHost(...) not closing the request properly

Fixes OfficeDev/ews-java-api#152.
perhabs the constants in [Strings](https://github.com/OfficeDev/ews-java-api/blob/master/src/main/java/microsoft/exchange/webservices/data/Strings.java#L104) may be used here (if keeping all Strings there is state of the art)?