Only remove explicitly deleted relationships

Only when a relationship is defined as a null JSON value, it should be
deleted. If it is missing, it should be ignored.

server/core/test/rest-api/test/server.js

@@ -68,8 +68,8 @@ describe("Server Relationships", function() {
     });
 
     it("remove relationships", function() {
-        delete rel_server.data.relationships["services"]
-        delete rel_server.data.relationships["monitors"]
+        rel_server.data.relationships["services"] = null
+        rel_server.data.relationships["monitors"] = null
         return request.patch(base_url + "/servers/" + rel_server.data.id, {json: rel_server})
             .should.be.fulfilled
     });