Added a content-type constant

Author: GrahamCampbell

src/Api/AbstractApi.php

@@ -271,7 +271,7 @@ private function createMultipartStreamBuilder(array $params = [], array $files =
         foreach ($files as $name => $file) {
             $builder->addResource($name, self::tryFopen($file, 'r'), [
                 'headers' => [
-                    'Content-Type' => self::guessFileContentType($file),
+                    ResponseMediator::CONTENT_TYPE_HEADER => self::guessFileContentType($file),
                 ],
                 'filename' => \basename($file),
             ]);
@@ -304,7 +304,7 @@ private static function addMultipartContentType(array $headers, MultipartStreamB
     {
         $contentType = \sprintf('%s; boundary=%s', ResponseMediator::MULTIPART_CONTENT_TYPE, $builder->getBoundary());
 
-        return \array_merge(['Content-Type' => $contentType], $headers);
+        return \array_merge([ResponseMediator::CONTENT_TYPE_HEADER => $contentType], $headers);
     }
 
     /**
@@ -336,7 +336,7 @@ private static function prepareJsonBody(array $params)
      */
     private static function addJsonContentType(array $headers)
     {
-        return \array_merge(['Content-Type' => ResponseMediator::JSON_CONTENT_TYPE], $headers);
+        return \array_merge([ResponseMediator::CONTENT_TYPE_HEADER => ResponseMediator::JSON_CONTENT_TYPE], $headers);
     }
 
     /**

src/HttpClient/Message/ResponseMediator.php

@@ -13,6 +13,13 @@
  */
 final class ResponseMediator
 {
+    /**
+     * The content type header.
+     *
+     * @var string
+     */
+    public const CONTENT_TYPE_HEADER = 'Content-Type';
+
     /**
      * The JSON content type identifier.
      *
@@ -45,7 +52,7 @@ public static function getContent(ResponseInterface $response)
     {
         $body = (string) $response->getBody();
 
-        if (in_array($body, ['', 'null', 'true', 'false'], true) && 0 === \strpos($response->getHeaderLine('Content-Type'), self::JSON_CONTENT_TYPE)) {
+        if (in_array($body, ['', 'null', 'true', 'false'], true) && 0 === \strpos($response->getHeaderLine(self::CONTENT_TYPE_HEADER), self::JSON_CONTENT_TYPE)) {
             return JsonArray::decode($body);
         }