Use class property instead of $current for proper semantics

$current is a 0-based array index whereas $this->current is the page number. Using $current introduces a bug, since it clears $this->prev on page 2.

src/Pagination.php

@@ -116,7 +116,7 @@ protected function init($prefs = [], $wp_query = null)
                 'class' => 'page-numbers prev',
             ];
         }
-        if ($current < 2) {
+        if ($this->current < 2) {
             $this->prev = '';
         }
         if ($this->total === (float) 0) {