Use charset UTF-8 for JSON responses

JSON is unicode, but if no charset is specified, ISO-8859-1 will be used
by servlet containers.

Author: JessThrysoee

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

@@ -281,7 +281,7 @@ public Void run() {
         } else {
             GraphQLVariables vars = new GraphQLVariables(schema, query, variables);
             ExecutionResult result = new GraphQL(schema, executionStrategyProvider.getExecutionStrategy()).execute(query, operationName, context, vars);
-            resp.setContentType("application/json");
+            resp.setContentType("application/json;charset=utf-8");
             if (result.getErrors().isEmpty()) {
                 Map<String, Object> dict = new HashMap<>();
                 dict.put("data", result.getData());