fixing some psalm 5 complaints (#1110)

Whilst investigating upgrading to psalm 5, I notice that it generates a lot of new complaints. This fixes the ones that looked legit and
require an API change, or were trivial. Does not upgrade to psalm 5 yet, though. That's a bigger job for another day.

Author: brettmc

Common/Adapter/HttpDiscovery/DependencyResolver.php

@@ -71,11 +71,6 @@ public function resolveUriFactory(): UriFactoryInterface
         return $this->messageFactoryResolver->resolveUriFactory();
     }
 
-    public function resolveHttpClient(): ClientInterface
-    {
-        return $this->psrClientResolver->resolvePsrClient();
-    }
-
     public function resolveHttpPlugAsyncClient(): HttpAsyncClient
     {
         return $this->httpPlugClientResolver->resolveHttpPlugAsyncClient();

Common/Export/Http/PsrTransportFactory.php

@@ -57,7 +57,7 @@ public function create(
             $endpoint,
             $contentType,
             $headers,
-            (array) $compression,
+            PsrUtils::compression($compression),
             $retryDelay,
             $maxRetries,
         );

Common/Export/Http/PsrUtils.php

@@ -104,6 +104,24 @@ public static function decode(string $value, array $encodings): string
         return $value;
     }
 
+    /**
+     * Resolve an array or CSV of compression types to a list
+     */
+    public static function compression($compression): array
+    {
+        if (is_array($compression)) {
+            return $compression;
+        }
+        if (!$compression) {
+            return [];
+        }
+        if (strpos($compression, ',') === false) {
+            return [$compression];
+        }
+
+        return array_map('trim', explode(',', $compression));
+    }
+
     private static function encoder(string $encoding): ?callable
     {
         static $encoders;