RemoteUserUtil: Don't use default platform charset when constructing String

Reported by Error Prone 2.0.15.

Change-Id: Ia16ed7a630ed854722c819a5f2cd244cb0aff290
diff --git a/gerrit-httpd/src/main/java/com/google/gerrit/httpd/RemoteUserUtil.java b/gerrit-httpd/src/main/java/com/google/gerrit/httpd/RemoteUserUtil.java
index 479a5e5..e99838a 100644
--- a/gerrit-httpd/src/main/java/com/google/gerrit/httpd/RemoteUserUtil.java
+++ b/gerrit-httpd/src/main/java/com/google/gerrit/httpd/RemoteUserUtil.java
@@ -16,6 +16,7 @@
 
 import static com.google.common.base.Strings.emptyToNull;
 import static com.google.common.net.HttpHeaders.AUTHORIZATION;
+import static java.nio.charset.StandardCharsets.UTF_8;
 
 import org.eclipse.jgit.util.Base64;
 
@@ -72,7 +73,7 @@
 
     } else if (auth.startsWith("Basic ")) {
       auth = auth.substring("Basic ".length());
-      auth = new String(Base64.decode(auth));
+      auth = new String(Base64.decode(auth), UTF_8);
       final int c = auth.indexOf(':');
       return c > 0 ? auth.substring(0, c) : null;