More fixes for PHP 7.4 compat

src/Readers/CsvReader.php

@@ -70,13 +70,13 @@ class CsvReader implements ReaderInterface
         $this->checkLoadedSlice();
     }
 
-    public function key(): mixed
+    public function key()
     {
         //$this->checkLoadedSlice();
         return $this->counter;
     }
 
-    public function current(): mixed
+    public function current()
     {
         //$this->checkLoadedSlice();   
         return $this->data[$this->currentIndex];

src/Readers/JsonReader.php

@@ -61,13 +61,13 @@ class JsonReader implements ReaderInterface
         $this->checkLoadedSlice();
     }
 
-    public function key(): mixed
+    public function key()
     {
         //$this->checkLoadedSlice();
         return $this->counter;
     }
 
-    public function current(): mixed
+    public function current()
     {
         //$this->checkLoadedSlice();   
         return $this->data[$this->currentIndex];