Add check so that getContents() always returns content with MimeType text/plain first in array of Content objects

lib/mail/Mail.php

@@ -1093,11 +1093,28 @@ public function addContents($contents)
 
     /**
      * Retrieve the contents attached to a Mail object
+     *
+     * Will return array of Content Objects with text/plain MimeType first
+     * Array re-ordered before return where this is not already the case
      * 
      * @return Content[]
      */  
     public function getContents()
     {
+        if ($this->contents[0]->getType() !== 'text/plain'
+            && count($this->contents) > 1
+        ) {
+
+            foreach ($this->contents as $key => $value) {
+                if ($value->getType() == 'text/plain') {
+                    $plain_content = $value;
+                    unset($this->contents[$key]);
+                    break;
+                }
+            }
+            array_unshift($this->contents, $plain_content);
+        }
+
         return $this->contents;
     }
 

test/unit/MailGetContentsTest.php

@@ -0,0 +1,39 @@
+<?php
+
+namespace SendGrid\Tests;
+
+use PHPUnit\Framework\TestCase;
+use SendGrid\Mail\Mail;
+use SendGrid\Mail\Content;
+use SendGrid\Mail\EmailAddress;
+use SendGrid\Mail\From;
+
+/**
+ * This class tests the getContents() function in SendGrid\Mail\Mail
+ *
+ * @package SendGrid\Tests
+ */
+class MailGetContentsTest extends TestCase
+{
+
+    /**
+     * This method tests that array from Mail getContents() returns with
+     * text/plain Content object first when Mail instantiated with text/html
+     * content before text/plain
+     *
+     * @return null
+     */
+    public function testWillReturnPlainContentFirst()
+    {
+        $from = new From(null, "test@example.com");
+        $subject = "Hello World from the SendGrid PHP Library";
+        $to = new EmailAddress("Test Person", "test@example.com");
+
+        $plain_content = new Content("text/plain", "some text here");
+        $html_content = new Content("text/html", "<p>some text here</p>");
+
+        $mail = new Mail($from, [$to], $subject, $html_content, $plain_content);
+
+        $this->assertEquals('text/plain', $mail->getContents()[0]->getType());
+    }
+}