Remove unused import && add return value (#60)

Author: kefzce

src/Document.php

@@ -74,21 +74,21 @@ public static function fromResources(ResourceObject ...$resources): self
         return $doc;
     }
 
-    public static function fromIdentifier(ResourceIdentifier $identifier)
+    public static function fromIdentifier(ResourceIdentifier $identifier): self
     {
         $doc = new self;
         $doc->data = new SingleIdentifierData($identifier);
         return $doc;
     }
 
-    public static function fromIdentifiers(ResourceIdentifier... $identifiers)
+    public static function fromIdentifiers(ResourceIdentifier... $identifiers): self
     {
         $doc = new self;
         $doc->data = new MultiIdentifierData(...$identifiers);
         return $doc;
     }
 
-    public static function nullDocument()
+    public static function nullDocument(): self
     {
         $doc = new self;
         $doc->data = new NullData();

test/Document/CompoundDocumentTest.php

@@ -4,7 +4,6 @@
 namespace JsonApiPhp\JsonApi\Test\Document;
 
 use JsonApiPhp\JsonApi\Document;
-use JsonApiPhp\JsonApi\Document\Link\Link;
 use JsonApiPhp\JsonApi\Document\Meta;
 use JsonApiPhp\JsonApi\Document\Resource\Linkage\MultiLinkage;
 use JsonApiPhp\JsonApi\Document\Resource\Linkage\SingleLinkage;