[5.8] Add getEndpoint and setEndpoint methods to the MailgunTransport class

[SerhiiStarovoitov]

If you need to switch (on the fly) your mailgun domain while sending emails you can do it by updating the MailgunTransport "domain" property using the setDomain() method. But if your mailgun domain region is EU, you should also change the endpoint property value from "api.mailgun.net" to "api.eu.mailgun.net".

This PR adds setEndpoint method to MailgunTransport class, so you can do it like this (in your Mailable-class, for example):

$mailTransport = app()->make('mailer')->getSwiftMailer()->getTransport();
$mailTransport->setDomain('yourdomain.com');
$mailTransport->setEndpoint('api.eu.mailgun.net');

[seriquynh]

I think a setter method should return void or $this

[SerhiiStarovoitov]

Yeah, but all the other setter methods (key and domain) return the property value, so I left it the same for the endpoint setter.

[SerhiiStarovoitov]

One more typo: #29329