Handle Carbon instances in message search criteria (#82)

Allow searching messages using a Carbon instance for the date criteria
(BEFORE, ON, SINCE)

Author: Stéphane Coinon

README.md

@@ -178,7 +178,7 @@ Search for specific emails:
 
 //Get all messages since march 15 2018
 /** @var \Webklex\IMAP\Support\MessageCollection $aMessage */
-$aMessage = $oFolder->searchMessages([['SINCE', Carbon::parse('15.03.2018')->format('d M y')]]);
+$aMessage = $oFolder->searchMessages([['SINCE', Carbon::parse('15.03.2018')]]);
 
 //Get all messages containing "hello world"
 /** @var \Webklex\IMAP\Support\MessageCollection $aMessage */

src/IMAP/Folder.php

@@ -211,7 +211,7 @@ public function getUnseenMessages($criteria = 'UNSEEN', $fetch_options = null, $
      *                        [['FROM' => '[email protected]'],[' TEXT' => 'Hello world!']]
      *                        ---------------------------------------------------------------------------------------
      *                        The following sample would search for all messages received since march 15 2018:
-     *                        [['SINCE' => Carbon::parse('15.03.2018')->format('d M y')]]
+     *                        [['SINCE' => Carbon::parse('15.03.2018')]]
      *                        ---------------------------------------------------------------------------------------
      *                        The following sample would search for all flagged messages:
      *                        [['FLAGGED']]
@@ -279,7 +279,11 @@ public function searchMessages(array $where, $fetch_options = null, $fetch_body
                 if (count($statement) == 1) {
                     $query .= $statement[0];
                 } else {
-                    $query .= $statement[0].' "'.$statement[1].'"';
+                    $value = $statement[1];
+                    if ($value instanceof \Carbon\Carbon) {
+                        $value = $value->format('d M y');
+                    }
+                    $query .= $statement[0].' "'.$value.'"';
                 }
             }