Make the context and execution provider references optional, as they have default implementations.

Author: sergehuber

src/main/java/graphql/servlet/GraphQLServlet.java

@@ -131,15 +131,15 @@ public String[] getMutations() {
     private GraphQLContextBuilder contextBuilder = new DefaultGraphQLContextBuilder();
     private ExecutionStrategyProvider executionStrategyProvider = new EnhancedExecutionStrategyProvider();
 
-    @Reference
+    @Reference(cardinality = ReferenceCardinality.OPTIONAL)
     public void setContextProvider(GraphQLContextBuilder contextBuilder) {
         this.contextBuilder = contextBuilder;
     }
     public void unsetContextProvider(GraphQLContextBuilder contextBuilder) {
         this.contextBuilder = new DefaultGraphQLContextBuilder();
     }
 
-    @Reference
+    @Reference(cardinality = ReferenceCardinality.OPTIONAL)
     public void setExecutionStrategyProvider(ExecutionStrategyProvider provider) {
         executionStrategyProvider = provider;
     }