null values can shadow parent context now. Improve test a little.

src/main/java/com/samskivert/mustache/BasicCollector.java

@@ -37,7 +37,11 @@ public class BasicCollector implements Mustache.Collector
 
     protected static final Mustache.VariableFetcher MAP_FETCHER = new Mustache.VariableFetcher() {
         public Object get (Object ctx, String name) throws Exception {
-            return ((Map<?,?>)ctx).get(name);
+            if (((Map<?,?>)ctx).containsKey(name)) {
+                return ((Map<?,?>)ctx).get(name);
+            } else {
+                return Template.NO_FETCHER_FOUND;
+            }
         }
     };
 

src/main/java/com/samskivert/mustache/Template.java

@@ -140,10 +140,6 @@ public class Template
             Object value = getValueIn(ctx.data, name, line);
             if (value == NO_FETCHER_FOUND) {
                 // preserve variableMissing
-            } else if (value == null) {
-                // we found a fetcher, and it returned null; so we keep searching our parents, but
-                // we won't freak out about a missing variable if we have a nullValue configured
-                variableMissing = false;
             } else {
                 return value;
             }