JAVA-406: Java driver does not accept empty keys

Author: agirbal

src/main/com/mongodb/DBApiLayer.java

@@ -315,7 +315,7 @@ public WriteResult update( DBObject query , DBObject o , boolean upsert , boolea
             if (o != null && !o.keySet().isEmpty()) {
                 // if 1st key doesn't start with $, then object will be inserted as is, need to check it
                 String key = o.keySet().iterator().next();
-                if (key.charAt(0) != '$')
+                if (!key.startsWith("$"))
                     _checkObject(o, false, false);
             }
 

src/main/com/mongodb/DBCollection.java

@@ -1078,7 +1078,7 @@ private void _checkKeys( DBObject o ) {
         for ( String s : o.keySet() ){
             if ( s.contains( "." ) )
                 throw new IllegalArgumentException( "fields stored in the db can't have . in them" );
-            if ( s.charAt( 0 ) == '$' )
+            if ( s.startsWith( "$" ) )
                 throw new IllegalArgumentException( "fields stored in the db can't start with '$'" );
 
             Object inner;