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