Fixed testing mailer with API key

[alanhartless]

Q	A
Bug fix?	y
New feature?	n
Related user documentation PR URL	na
Related developer documentation PR URL	na
Issues addressed (#s or URLs)	#2535
BC breaks?	n
Deprecations?	n

Description:

is_callable had the mailer and method set as arguments rather than array and thus didn't properly check if setApiKey was callable leading to the Cannot create instance of \SparkPost\SparkPost while API key is NULL Log data: !! Cannot create instance of \SparkPost\SparkPost while API key is NULL (code: 0) error. Although some instances seem to still return successful tests.

Steps to test this PR:

1. Use sparkpost transport in Email Configuration
2. Add api key
3. Click test connection

Steps to reproduce the bug:

1. Repeat above and may get the error noted

[escopecz]

I don't have a Sparkpost key to test with, but the PHP docs say that it is possible to use an array as the function attribute:

This can check that a simple variable contains the name of a valid function, or that an array contains a properly encoded object and function name.

👍

[virlatinus]

Tested and it worked!