Add spec for empty string (issue #94)

spec/_files/empty_string.js

@@ -0,0 +1,6 @@
+var empty_string = {
+  description: "That is all!",
+  child: {
+    description: ""
+  }
+};

spec/_files/empty_string.mustache

@@ -0,0 +1 @@
+{{description}}{{#child}}{{description}}{{/child}}

spec/_files/empty_string.txt

@@ -0,0 +1 @@
+That is all!