Allow access coercion to be disabled.

I'm not disabling it by default because that would change behavior for anyone
who was using reflection, which seems like annoying breakage. People who are
using JMustache with Java 9 can configure a non-coercing DefaultCollector to
eliminate the illegal access warnings.

src/test/java/com/samskivert/mustache/MustacheTest.java

@@ -275,4 +275,22 @@ public class MustacheTest extends SharedTests
         test("k1v1k2v2", "{{#map.entrySet}}{{key}}{{value}}{{/map.entrySet}}",
              context("map", data));
     }
+
+    @Test public void testNoAccesCoercion () {
+        Object ctx = new Object() {
+            public Object foo () {
+                return new Object() {
+                    public Object bar = new Object() {
+                        public String baz = "hello";
+                        private String quux = "hello";
+                    };
+                };
+            }
+        };
+        test("hello:hello", "{{foo.bar.baz}}:{{foo.bar.quux}}", ctx);
+        Mustache.Compiler comp = Mustache.compiler().
+            withCollector(new DefaultCollector(false)).
+            defaultValue("missing");
+        test(comp, "hello:missing", "{{foo.bar.baz}}:{{foo.bar.quux}}", ctx);
+    }
 }