NullPointerException for HEAD request with HttpClient connector

[olivierlemasle]

Using the Apache HttpClient connector, a NullPointerException is thrown if the response contains no entity (which is the case for a HEAD request).

I encountered the issue with client.identity().roles().checkProjectUserRole() which is maped to a HEAD request (http://developer.openstack.org/api-ref/identity/v3/index.html?expanded=check-whether-user-has-role-assignment-on-project-detail,check-whether-group-has-role-assignment-on-project-detail#check-whether-group-has-role-assignment-on-project).

Then, when the HTTP response is mapped to an ActionResponse in https://github.com/ContainX/openstack4j/blob/3.0.2/core/src/main/java/org/openstack4j/core/transport/functions/ResponseToActionResponse.java#L31, the entity is null in https://github.com/ContainX/openstack4j/blob/3.0.2/connectors/httpclient/src/main/java/org/openstack4j/connectors/httpclient/HttpResponseImpl.java#L127, which causes the NullPointerException.

java.lang.NullPointerException
    at org.openstack4j.connectors.httpclient.HttpResponseImpl.readEntity(HttpResponseImpl.java:127) [openstack4j-httpclient-3.0.2.jar:?]
    at org.openstack4j.core.transport.functions.ResponseToActionResponse.apply(ResponseToActionResponse.java:31) [openstack4j-core-3.0.2.jar:?]
    at org.openstack4j.core.transport.functions.ResponseToActionResponse.apply(ResponseToActionResponse.java:22) [openstack4j-core-3.0.2.jar:?]
    at org.openstack4j.core.transport.HttpEntityHandler.handle(HttpEntityHandler.java:36) [openstack4j-core-3.0.2.jar:?]
    at org.openstack4j.connectors.httpclient.HttpResponseImpl.getEntity(HttpResponseImpl.java:65) [openstack4j-httpclient-3.0.2.jar:?]
    at org.openstack4j.openstack.internal.BaseOpenStackService$Invocation.execute(BaseOpenStackService.java:193) [openstack4j-core-3.0.2.jar:?]
    at org.openstack4j.openstack.internal.BaseOpenStackService$Invocation.execute(BaseOpenStackService.java:187) [openstack4j-core-3.0.2.jar:?]
    at org.openstack4j.openstack.identity.v3.internal.RoleServiceImpl.checkProjectUserRole(RoleServiceImpl.java:53) [openstack4j-core-3.0.2.jar:?]