Email: Fix handling multiple recipients

src/Email.php

@@ -125,9 +125,7 @@ class Email {
 
     $headers["To"] = $to;
 
-    $to = array (
-      'To' => $to
-    );
+    $to = ensure_is_array($to);
 
     foreach(array_keys($headers) as $header) {
       if(in_array(strtoupper($header), array('BCC', 'CC'))) {
@@ -144,7 +142,7 @@ class Email {
           unset($headers[$header]);
           continue;
         }
-        $to[strtoupper($header)] = $headers[$header];
+        $to = array_merge($to, $headers[$header]);
       }
     }