Test if headers return last added header value

CHANGELOG.md

@@ -25,6 +25,10 @@ All notable changes to this project will be documented in this file, in reverse
 
 ### Fixed
 
+- [#140](https://github.com/zendframework/zend-http/pull/140) fixes retrieval of headers when multiple headers of the same name
+  are added to the `Headers` instance; it now ensures that the last header added of the same
+  type is retrieved when it is not a multi-value type. Previous values are overwritten.
+
 - [#112](https://github.com/zendframework/zend-http/pull/112) provides performance improvements when parsing large chunked messages.
 
 - introduces changes to `Response::fromString()` to pull the next line of the response

src/Headers.php

@@ -213,8 +213,27 @@ public function addHeaderLine($headerFieldNameOrLine, $fieldValue = null)
      */
     public function addHeader(Header\HeaderInterface $header)
     {
-        $this->headersKeys[] = static::createKey($header->getFieldName());
-        $this->headers[]     = $header;
+        $key = static::createKey($header->getFieldName());
+        $index = array_search($key, $this->headersKeys);
+
+        // No header by that key presently; append key and header to list.
+        if ($index === false) {
+            $this->headersKeys[] = $key;
+            $this->headers[]     = $header;
+            return $this;
+        }
+
+        // Header exists, and is a multi-value header; append key and header to
+        // list (as multi-value headers are aggregated on retrieval)
+        $class = ($this->getPluginClassLoader()->load(str_replace('-', '', $key))) ?: Header\GenericHeader::class;
+        if (in_array(Header\MultipleHeaderInterface::class, class_implements($class, true))) {
+            $this->headersKeys[] = $key;
+            $this->headers[] = $header;
+            return $this;
+        }
+
+        // Otherwise, we replace the current instance.
+        $this->headers[$index] = $header;
 
         return $this;
     }
@@ -286,6 +305,7 @@ public function get($name)
         if (is_array($this->headers[$index])) {
             return $this->lazyLoadHeader($index);
         }
+
         return $this->headers[$index];
     }
 

test/HeadersTest.php

@@ -119,6 +119,17 @@ public function testHeadersHasAndGetWorkProperly()
         $this->assertSame($f, $headers->get('foo'));
     }
 
+    public function testHeadersGetReturnsLastAddedHeaderValue()
+    {
+        $headers = new Headers();
+        $headers->addHeaders([
+            new Header\GenericHeader('Foo', 'bar'),
+        ]);
+        $headers->addHeader(new Header\GenericHeader('Foo', $value = 'baz'));
+
+        $this->assertEquals($value, $headers->get('foo')->getFieldValue());
+    }
+
     public function testHeadersAggregatesHeaderObjects()
     {
         $fakeHeader = new Header\GenericHeader('Fake', 'bar');