Ruleset: allow for extending pre-existing array values

As things were, using `extend="true"` when setting an array property in a ruleset XML file could only extend another array property setting set in an (included) ruleset file.

A potential default array value set on the sniff itself could not be extended.

This commit changes this behaviour.
If a default array value is defined on a sniff, setting additional array values via a ruleset with `extend="true"` will now add these values to the pre-existing sniff property default value.

This also now allows for redefining the array _value_ for a predefined associative key.

Includes plenty of tests documenting and safeguarding the feature.

Fixes 15

Author: jrfnl

src/Ruleset.php

@@ -1669,6 +1669,7 @@ public function setSniffProperty($sniffClass, $name, $settings)
             && $settings['extend'] === true
             && isset($sniffObject->$propertyName) === true
             && is_array($sniffObject->$propertyName) === true
+            && is_array($value) === true
         ) {
             $sniffObject->$propertyName = array_merge($sniffObject->$propertyName, $value);
         } else {

tests/Core/Ruleset/Fixtures/TestStandard/Sniffs/SetProperty/PropertyTypeHandlingSniff.php

@@ -239,6 +239,13 @@ final class PropertyTypeHandlingSniff implements Sniff
         'predefinedB' => '  null  ',
     ];
 
+    /**
+     * Used to verify that if `extend` is used on a non-array property, the value still gets set, but not as an array.
+     *
+     * @var array<mixed>
+     */
+    public $expectsStringNotArray;
+
     public function register()
     {
         return [T_ECHO];

tests/Core/Ruleset/PropertyTypeHandlingTest.php

@@ -277,6 +277,10 @@ public static function dataArrayPropertyExtending()
                     true,
                 ],
             ],
+            'Invalid "extend" used on a non-array property'                     => [
+                'propertyName' => 'expectsStringNotArray',
+                'expected'     => 'some value',
+            ],
         ];
 
     }//end dataArrayPropertyExtending()

tests/Core/Ruleset/PropertyTypeHandlingTest.xml

@@ -108,6 +108,8 @@
                 <element value="true"/>
             </property>
 
+            <property name="expectsStringNotArray" extend="true" value="some value"/>
+
         </properties>
     </rule>