Fix updating context inside custom helpers

Resolves zordius/lightncandy#350

Author: theodorejb

src/Compiler.php

@@ -91,6 +91,7 @@ public function composePHPRender(string $code): string
                     partials: isset(\$options['partials']) ? array_merge(\$partials, \$options['partials']) : \$partials,
                     data: isset(\$options['data']) ? array_merge(['root' => \$in], \$options['data']) : ['root' => \$in],
                 );
+                \$in = &\$cx->data['root'];
                 return '$code';
             };
             VAREND;

tests/RegressionTest.php

@@ -1545,6 +1545,24 @@ public static function issueProvider(): array
                 'expected' => ' {{bar}}  content',
             ],
 
+            [
+                'id' => 350,
+                'template' => <<<_hbs
+                    Before: {{var}}
+                    (Setting Variable) {{setvar "var" "Foo"}}
+                    After: {{var}}
+                    _hbs,
+                'data' => ['var' => 'value'],
+                'options' => new Options(
+                    helpers: [
+                        'setvar' => function ($name, $value, HelperOptions $options) {
+                            $options->data['root'][$name] = $value;
+                        },
+                    ],
+                ),
+                'expected' => "Before: value\n(Setting Variable) \nAfter: Foo",
+            ],
+
             [
                 'id' => 357,
                 'template' => '{{echo (echo "foobar(moo).")}}',