Fix the bug that Chinese characters are malformed in response body of ZuulBlockFallbackProvider (#737)

sentinel-adapter/sentinel-zuul-adapter/src/main/java/com/alibaba/csp/sentinel/adapter/gateway/zuul/filters/SentinelZuulPreFilter.java

@@ -133,6 +133,8 @@ public class SentinelZuulPreFilter extends ZuulFilter {
             // Set fallback response.
             ctx.setResponseBody(blockResponse.toString());
             ctx.setResponseStatusCode(blockResponse.getCode());
+            // Set Response ContentType
+            ctx.getResponse().setContentType("application/json; charset=utf-8");
         } finally {
             // We don't exit the entry here. We need to exit the entries in post filter to record Rt correctly.
             // So here the entries will be carried in the request context.

sentinel-demo/sentinel-demo-zuul-gateway/src/main/java/com/alibaba/csp/sentinel/demo/zuul/gateway/ZuulConfig.java

@@ -43,4 +43,4 @@ public class ZuulConfig {
     public ZuulFilter sentinelZuulErrorFilter() {
         return new SentinelZuulErrorFilter(-1);
     }
-}
+}