GetCapabilities#CheckOne: Return json content type

This makes the API consistent with the GetActive endpoint which
returns json content type.

Change-Id: Ic9ff09a4dc9572f5c6b7a1056bee51cb7602e0d0
diff --git a/Documentation/rest-api-accounts.txt b/Documentation/rest-api-accounts.txt
index 95dd4f2..6cb6e24 100644
--- a/Documentation/rest-api-accounts.txt
+++ b/Documentation/rest-api-accounts.txt
@@ -1089,7 +1089,10 @@
 .Response
 ----
   HTTP/1.1 200 OK
+  Content-Disposition: attachment
+  Content-Type: application/json; charset=UTF-8
 
+  )]}'
   ok
 ----
 
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/account/GetCapabilities.java b/gerrit-server/src/main/java/com/google/gerrit/server/account/GetCapabilities.java
index 4058a16..0232bf02 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/account/GetCapabilities.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/account/GetCapabilities.java
@@ -24,7 +24,7 @@
 import com.google.gerrit.extensions.config.CapabilityDefinition;
 import com.google.gerrit.extensions.registration.DynamicMap;
 import com.google.gerrit.extensions.restapi.AuthException;
-import com.google.gerrit.extensions.restapi.BinaryResult;
+import com.google.gerrit.extensions.restapi.Response;
 import com.google.gerrit.extensions.restapi.RestReadView;
 import com.google.gerrit.server.CurrentUser;
 import com.google.gerrit.server.OptionUtil;
@@ -157,8 +157,8 @@
   @Singleton
   static class CheckOne implements RestReadView<AccountResource.Capability> {
     @Override
-    public BinaryResult apply(Capability resource) {
-      return BinaryResult.create("ok\n");
+    public Response<String> apply(Capability resource) {
+      return Response.ok("ok");
     }
   }
 }