Unresolved reference while instantiating Encoding

If there's a `$ref` within a `multipart/form-data` `requestBody`, the `Encoding` will be instantiated with a `Reference` object instead of a `Schema` object as expected resulting in a `TypeErrorException`.
Since this parameter is only ever used to guess the default encoding type of a property if and when it is not explicitly defined, I propose to skip the schema parameter altogether if during instantiation it is still an unresolved reference (otherwise we would have to pre-resolve references during instantiation leading to all other kinds of mishaps and a very substantial rewrite of the whole project).

Author: karsa-mistmere

src/spec/MediaType.php

@@ -53,7 +53,13 @@ public function __construct(array $data)
                 if ($encodingData instanceof Encoding) {
                     $encoding[$property] = $encodingData;
                 } elseif (is_array($encodingData)) {
-                    $encoding[$property] = new Encoding($encodingData, $this->schema->properties[$property] ?? null);
+                    // Don't pass the schema if it's still an unresolved reference.
+                    if (this->schema->properties[$property] instanceof Reference) {
+                        $encoding[$property] = new Encoding($encodingData);
+                    }
+                    else {
+                        $encoding[$property] = new Encoding($encodingData, $this->schema->properties[$property] ?? null);
+                    }
                 } else {
                     $givenType = gettype($encodingData);
                     if ($givenType === 'object') {