Merge branch 'hotfix/#5957-php-5.6-compatibility'

Close #5957
Failing test for #6132 - form element manager calls `init()` on shared form elements multiple times
Added check for whether or not a plugin has been initialized from the FormElementManager
Only do the initialized check if shared
check if key exists instead of value
made $initializedPlugins an array by default
Refactored to use initializor
Cleanup
Merge pull request #1 from Ocramius/test/#6132-form-element-manager-multiple-shared-element-init

Tests for #6132 form element manager multiple shared element init
Merge branch 'hotfix/#6132-form-element-manager-only-initialize-shared-elements-once' into develop

Close #6132
Forward Port #6132