Add includeZero flag to if conditional

Allows for users who desire non-falsy handling of numbers to utilize if while maintaining the legacy if behavior.

Fixes #608

lib/handlebars/base.js

@@ -133,15 +133,18 @@ function registerDefaultHelpers(instance) {
   instance.registerHelper('if', function(conditional, options) {
     if (isFunction(conditional)) { conditional = conditional.call(this); }
 
-    if (Utils.isEmpty(conditional)) {
+    // Default behavior is to render the positive path if the value is truthy and not empty.
+    // The `includeZero` option may be set to treat the condtional as purely not empty based on the
+    // behavior of isEmpty. Effectively this determines if 0 is handled by the positive path or negative.
+    if ((!options.hash.includeZero && !conditional) || Utils.isEmpty(conditional)) {
       return options.inverse(this);
     } else {
       return options.fn(this);
     }
   });
 
   instance.registerHelper('unless', function(conditional, options) {
-    return instance.helpers['if'].call(this, conditional, {fn: options.inverse, inverse: options.fn});
+    return instance.helpers['if'].call(this, conditional, {fn: options.inverse, inverse: options.fn, hash: options.hash});
   });
 
   instance.registerHelper('with', function(context, options) {

spec/builtins.js

@@ -15,8 +15,11 @@ describe('builtin helpers', function() {
                       "if with non-empty array shows the contents");
       shouldCompileTo(string, {goodbye: [], world: "world"}, "cruel world!",
                       "if with empty array does not show the contents");
-      shouldCompileTo(string, {goodbye: 0, world: "world"}, "GOODBYE cruel world!",
-                      "if with zero does show the contents");
+      shouldCompileTo(string, {goodbye: 0, world: "world"}, "cruel world!",
+                      "if with zero does not show the contents");
+      shouldCompileTo("{{#if goodbye includeZero=true}}GOODBYE {{/if}}cruel {{world}}!",
+                      {goodbye: 0, world: "world"}, "GOODBYE cruel world!",
+                      "if with zero does not show the contents");
     });
 
     it("if with function argument", function() {