Addressed review feedback: used HttpHeaders.CONTENT_TYPE

Author: Behnam Mozafari

src/main/java/com/uid2/core/vertx/CoreVerticle.java

@@ -212,7 +212,7 @@ private void handleGetConfig(RoutingContext rc) {
                 String fileContent = ar.result().toString();
                 JsonObject configJson = new JsonObject(fileContent);
                 rc.response()
-                        .putHeader("content-type", "application/json")
+                        .putHeader(HttpHeaders.CONTENT_TYPE, "application/json")
                         .end(configJson.encodePrettily());
             } else {
                 rc.response()

src/test/java/com/uid2/core/vertx/TestCoreVerticle.java

@@ -17,6 +17,7 @@
 import com.uid2.shared.store.reader.RotatingCloudEncryptionKeyProvider;
 import io.vertx.core.*;
 import io.vertx.core.buffer.Buffer;
+import io.vertx.core.http.HttpHeaders;
 import io.vertx.core.json.JsonArray;
 import io.vertx.core.json.JsonObject;
 import io.vertx.ext.web.client.HttpResponse;
@@ -889,7 +890,7 @@ void getConfigSuccess(Vertx vertx, VertxTestContext testContext) throws Exceptio
                     System.out.println("Response: " + response.bodyAsString());
                     // Validate response
                     assertEquals(200, response.statusCode());
-                    assertEquals("application/json", response.getHeader("content-type"));
+                    assertEquals("application/json", response.getHeader(HttpHeaders.CONTENT_TYPE));
                     JsonObject actualConfig = new JsonObject(response.bodyAsString());
                     assertEquals(expectedConfig, actualConfig);
                     testContext.completeNow();