Add reply_to to helpers.Mail

Otherwise, we must create the Mail and then set the reply_to thanks to
its setter.
(and we have to dig the code to find out).

sendgrid/helpers/mail/mail.py

@@ -24,6 +24,7 @@ def __init__(
             self,
             from_email=None,
             to_emails=None,
+            reply_to=None,
             subject=None,
             plain_text_content=None,
             html_content=None,
@@ -40,6 +41,8 @@ def __init__(
         :param to_emails: The email address of the recipient
         :type to_emails: To, str, tuple, list(str), list(tuple),
                          list(To), optional
+        :param reply_to: The email address to reply to
+        :type reply_to: ReplyTo, tuple, optional
         :param plain_text_content: The plain text body of the email
         :type plain_text_content: string, optional
         :param html_content: The html body of the email
@@ -79,6 +82,10 @@ def __init__(
         if html_content is not None:
             self.add_content(html_content, MimeType.html)
 
+        # Optional
+        if reply_to is not None:
+            self.reply_to = reply_to
+
     def __str__(self):
         """A JSON-ready string representation of this Mail object.
 

test/test_mail_helpers.py

@@ -100,6 +100,7 @@ def test_single_email_to_a_single_recipient(self):
         message = Mail(
             from_email=From('[email protected]', 'Example From Name'),
             to_emails=To('[email protected]', 'Example To Name'),
+            reply_to=ReplyTo('[email protected]', 'Example Reply To Name'),
             subject=Subject('Sending with SendGrid is Fun'),
             plain_text_content=PlainTextContent(
                 'and easy to do anywhere, even with Python'),
@@ -123,6 +124,10 @@ def test_single_email_to_a_single_recipient(self):
                     "email": "[email protected]",
                     "name": "Example From Name"
                 },
+                "reply_to": {
+                    "email": "[email protected]",
+                    "name": "Example Reply To Name"
+                },
                 "personalizations": [
                     {
                         "to": [