Fix GroupsToDisplay::setGroupsToDisplay's handling of array arguments

[moismailzai]

Issue Summary

According to the API docs, asm.groups_to_display should be an array of integers.

Several of the examples in this repository show that sendgrid-php is meant to support passing an array to asm.groups_to_display:

• https://github.com/sendgrid/sendgrid-php/blob/master/USE_CASES.md references $email->setAsm(1, [1, 2, 3, 4]). The expectation is that the setAsm method would pass its second argument to the groups_to_display setter.
• https://github.com/sendgrid/sendgrid-php/blob/master/examples/helpers/mail/example.php#L117 references $asm->setGroupsToDisplay([4,5,6,7,8]). The expectation is that this array would be set as the asm.groups_to_display.

However, both of these examples return 400 with:
{"errors":[{"message":"Invalid type. Expected: integer, given: array.","field":"asm.groups_to_display.0","help":null}]}

This because the setGroupsToDisplay method of the GroupsToDisplay class is wrapping its argument ($groups_to_display) in an array: $this->groups_to_display[] = $groups_to_display;. This is necessary when $groups_to_display is a single integer, but it breaks when $groups_to_display is already an array. The fix is a conditional that checks if $groups_to_display is an array.

Steps to Reproduce

1. Create a new mail object $email = new \SendGrid\Mail\Mail() and setAsm with two arguments, an integer and an array of integers: $email->setAsm(1, array( 2, 3, 4)).
2. Send the mail and you will get a 400 response citing asm.groups_to_display.

Alternatively,

1. Create a new mail object and configure your ASM like below:

$asm = new \SendGrid\Mail\Asm();
$asm->setGroupId(1);
$asm->setGroupsToDisplay(array(2, 3, 4));
...
$email->setAsm($asm);

2. Send the mail and you will get a 400 response citing asm.groups_to_display.

Technical details:

• sendgrid-php Version: master (latest commit: a3da5cdb59e60bd692325ff286d68095a03bd928)
• PHP Version: 7.1, 7.2

[thinkingserious]

Thank you @moismailzai!