WebServer: Fix TIME_IN_THE_PAST constant definition

January 1st 1990 was a Monday, not Sunday [1].

[1] http://www.timeanddate.com/calendar/?year=1990&country=1

Change-Id: Ide8366a9dbc4f82404ebacc7d711573cb7f3f30e
diff --git a/gerrit-gwtdebug/src/main/java/com/google/gwt/dev/codeserver/WebServer.java b/gerrit-gwtdebug/src/main/java/com/google/gwt/dev/codeserver/WebServer.java
index 8072d75..728f276 100644
--- a/gerrit-gwtdebug/src/main/java/com/google/gwt/dev/codeserver/WebServer.java
+++ b/gerrit-gwtdebug/src/main/java/com/google/gwt/dev/codeserver/WebServer.java
@@ -91,7 +91,7 @@
 
   private static final MimeTypes MIME_TYPES = new MimeTypes();
 
-  private static final String TIME_IN_THE_PAST = "Fri, 01 Jan 1990 00:00:00 GMT";
+  private static final String TIME_IN_THE_PAST = "Mon, 01 Jan 1990 00:00:00 GMT";
 
   private final SourceHandler handler;
   private final JsonExporter jsonExporter;