Replaces deprecated getMock with createMock
Replaces deprecated getMock with createMock

As of PHPUnit 5.4.0 (released more than a year ago) `getMock` has been deprecated, `createMock` is the correct method to use.

https://github.com/sebastianbergmann/phpunit/wiki/Release-Announcement-for-PHPUnit-5.4.0
php-http/documentation,php-http/documentation,php-http/documentation