Allows variables that evaluate to falsy to be returned (#23)

The current code skips particular scopes that have a variable defined
when the variable is falsy. This is no good as legitimate values such
as empty string and boolean 'false' are then skipped and end up being
set to undefined.

lib/shim/core.js

@@ -527,14 +527,23 @@ const pm = Object.freeze({
   /* General variable access */
   variables: Object.freeze({
     get(name) {
-      return (
-        (scope.local[Has](name) && scope.local[name]) ||
-        (scope.data[Has](name) && scope.data[name]) ||
-        (scope.environment[Has](name) && scope.environment[name]) ||
-        (scope.collection[Has](name) && scope.collection[name]) ||
-        (scope.global[Has](name) && scope.global[name]) ||
-        undef
-      );
+      if (scope.local[Has](name)) {
+        return scope.local[name];
+      }
+      if (scope.data[Has](name)) {
+        return scope.data[name];
+      }
+      if (scope.environment[Has](name)) {
+        return scope.environment[name];
+      }
+      if (scope.collection[Has](name)) {
+        return scope.collection[name];
+      }
+      if (scope.global[Has](name)) {
+        return scope.global[name];
+      }
+
+      return undef;
     },
     set(name, value) {
       requireRequest();