Fix linting for spacing around default parameter assignment in function/method declaration

Summary: Fix linting for spacing around default parameter assignment in function/method declaration so that `function foo($x = null) {}` is preferred in favor of `function foo($x=null) {}`.

Test Plan: Modified existing unit tests.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10429