RestSession: Use StringEntity(String, Charset)

Pass the Charset rather than Charset.name().

Change-Id: I82ebfec4db251e431c03a9fed64d771652ab9989
diff --git a/gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/RestSession.java b/gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/RestSession.java
index eb8e2b7..aee629d 100644
--- a/gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/RestSession.java
+++ b/gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/RestSession.java
@@ -79,7 +79,7 @@
       put.addHeader(new BasicHeader("Content-Type", "application/json"));
       put.body(new StringEntity(
           OutputFormat.JSON_COMPACT.newGson().toJson(content),
-          UTF_8.name()));
+          UTF_8));
     }
     return execute(put);
   }
@@ -105,7 +105,7 @@
       post.addHeader(new BasicHeader("Content-Type", "application/json"));
       post.body(new StringEntity(
           OutputFormat.JSON_COMPACT.newGson().toJson(content),
-          UTF_8.name()));
+          UTF_8));
     }
     return execute(post);
   }