Make #each handle non-iterable values like the original JS library.

Previously things like `{{#each "foo"}}` were render errors. Now
non-iterable values are treated like empty iterables. This brings
handlebars-rust's behavior closer in line with the original JavaScript
library.

Here are the two new test cases in the handlebars playground:

1. https://handlebarsjs.com/playground.html#format=1&currentExample=%7B%22template%22%3A%22%7B%7B%23each%20this%7D%7Deach%20block%7B%7Belse%7D%7Delse%20block%7B%7B%2Feach%7D%7D%22%2C%22partials%22%3A%5B%5D%2C%22input%22%3A%22%5C%22strings%20aren't%20iterable%5C%22%22%2C%22output%22%3A%22else%20block%22%2C%22preparationScript%22%3A%22%22%2C%22handlebarsVersion%22%3A%224.7.6%22%7D

2. https://handlebarsjs.com/playground.html#format=1&currentExample=%7B%22template%22%3A%22%7B%7B%23*inline%20%5C%22walk%5C%22%7D%7D(%7B%7B%23each%20this%7D%7D%7B%7B%23if%20%40key%7D%7D%7B%7B%40key%7D%7D%7B%7Belse%7D%7D%7B%7B%40index%7D%7D%7B%7B%2Fif%7D%7D%3A%20%7B%7Bthis%7D%7D%20%7B%7B%3E%20walk%20this%7D%7D%2C%20%7B%7B%2Feach%7D%7D)%7B%7B%2Finline%7D%7D%5Cn%7B%7B%3E%20walk%7D%7D%22%2C%22partials%22%3A%5B%5D%2C%22input%22%3A%22%7B%5Cn%20%20%20%20%5C%22array%5C%22%3A%20%5B42%2C%20%7B%5C%22wow%5C%22%3A%20%5C%22cool%5C%22%7D%2C%20%5B%5B%5D%5D%5D%2C%5Cn%20%20%20%20%5C%22object%5C%22%3A%20%7B%20%5C%22a%5C%22%3A%20%7B%20%5C%22b%5C%22%3A%20%5C%22c%5C%22%2C%20%5C%22d%5C%22%3A%20%5B%5C%22e%5C%22%5D%20%7D%20%7D%2C%5Cn%20%20%20%20%5C%22string%5C%22%3A%20%5C%22hi%5C%22%5Cn%7D%22%2C%22output%22%3A%22%5Cn(array%3A%2042%2C%5Bobject%20Object%5D%2C%20(0%3A%2042%20()%2C%201%3A%20%5Bobject%20Object%5D%20(wow%3A%20cool%20()%2C%20)%2C%202%3A%20%20(0%3A%20%20()%2C%20)%2C%20)%2C%20object%3A%20%5Bobject%20Object%5D%20(a%3A%20%5Bobject%20Object%5D%20(b%3A%20c%20()%2C%20d%3A%20e%20(0%3A%20e%20()%2C%20)%2C%20)%2C%20)%2C%20string%3A%20hi%20()%2C%20)%22%2C%22preparationScript%22%3A%22%22%2C%22handlebarsVersion%22%3A%224.7.6%22%7D

src/helpers/helper_each.rs

@@ -136,16 +136,12 @@ impl HelperDef for EachHelper {
                     rc.pop_block();
                     Ok(())
                 }
-                (false, _) => {
+                _ => {
                     if let Some(else_template) = h.inverse() {
                         else_template.render(r, ctx, rc, out)?;
                     }
                     Ok(())
                 }
-                _ => Err(RenderError::new(format!(
-                    "Param type is not iterable: {:?}",
-                    value.value()
-                ))),
             },
             None => Ok(()),
         }
@@ -479,4 +475,53 @@ mod test {
         let rendered = reg.render_template(template, &input).unwrap();
         assert_eq!("01", rendered);
     }
+
+    #[test]
+    fn test_non_iterable() {
+        let reg = Registry::new();
+        let template = "{{#each this}}each block{{else}}else block{{/each}}";
+        let input = json!("strings aren't iterable");
+        let rendered = reg.render_template(template, &input).unwrap();
+        assert_eq!("else block", rendered);
+    }
+
+    #[test]
+    fn test_recursion() {
+        let mut reg = Registry::new();
+        assert!(reg
+            .register_template_string(
+                "walk",
+                "(\
+                    {{#each this}}\
+                        {{#if @key}}{{@key}}{{else}}{{@index}}{{/if}}: \
+                        {{this}} \
+                        {{> walk this}}, \
+                    {{/each}}\
+                )",
+            )
+            .is_ok());
+
+        let input = json!({
+            "array": [42, {"wow": "cool"}, [[]]],
+            "object": { "a": { "b": "c", "d": ["e"] } },
+            "string": "hi"
+        });
+        let expected_output = "(\
+            array: [42, [object], [[], ], ] (\
+                0: 42 (), \
+                1: [object] (wow: cool (), ), \
+                2: [[], ] (0: [] (), ), \
+            ), \
+            object: [object] (\
+                a: [object] (\
+                    b: c (), \
+                    d: [e, ] (0: e (), ), \
+                ), \
+            ), \
+            string: hi (), \
+        )";
+
+        let rendered = reg.render("walk", &input).unwrap();
+        assert_eq!(expected_output, rendered);
+    }
 }