fix: correct merging of contexts with targetingKey (#136)

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## This PR
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Corrects the behavior of EvaluationContextMerger `merge()` when using
string `targetingKey`.

### Related Issues
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Fixes #135 

### Notes
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### Follow-up Tasks
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### How to test
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The `testEvaluationContextMergingTargetingKey` test in
EvaluationContextTest.php has been added. The test fails without the
proposed fix, as expected.

Signed-off-by: Tommaso Tofacchi <tofacchitommaso@gmail.com>

src/implementation/flags/EvaluationContextMerger.php

@@ -47,10 +47,10 @@ public static function merge(?EvaluationContextInterface ...$contexts): Evaluati
 
                 /** @var ?string $newTargetingKey */
                 $newTargetingKey = null;
-                if (!is_null($calculatedTargetingKey) && strlen($calculatedTargetingKey) > 0) {
-                    $newTargetingKey = $calculatedTargetingKey;
-                } elseif (!is_null($additionalTargetingKey) && strlen($additionalTargetingKey) > 0) {
+                if (!is_null($additionalTargetingKey) && strlen($additionalTargetingKey) > 0) {
                     $newTargetingKey = $additionalTargetingKey;
+                } elseif (!is_null($calculatedTargetingKey) && strlen($calculatedTargetingKey) > 0) {
+                    $newTargetingKey = $calculatedTargetingKey;
                 }
 
                 $mergedAttributes = AttributesMerger::merge(

tests/unit/EvaluationContextTest.php

@@ -127,4 +127,17 @@ public function testEvaluationContextMerging(): void
 
         $this->assertEquals($expectedEvaluationContextAttributes, $actualEvaluationContextAttributes);
     }
+
+    public function testEvaluationContextMergingTargetingKey(): void
+    {
+        $firstEvaluationContext = new EvaluationContext('default');
+        $secondEvaluationContext = new EvaluationContext('merged_key');
+
+        $expectedEvaluationContextAttributes = 'merged_key';
+
+        $actualEvaluationContext = EvaluationContext::merge($firstEvaluationContext, $secondEvaluationContext);
+        $actualEvaluationContextAttributes = $actualEvaluationContext->getTargetingKey();
+
+        $this->assertEquals($expectedEvaluationContextAttributes, $actualEvaluationContextAttributes);
+    }
 }