Update DefaultValueValidator.php

Allow the default value to be a closure
Update CHANGELOG.md
Update DefaultValueValidator.php
Check if property is set

```getValue``` didn't check if the property on an object exists so wouldn't return the default value.
Update rest-quick-start.md: highlight automatic pluralization

This is probably not expected given the other naming conventions within Yii, so highlight this feature for beginners.
Update rest-quick-start.md: grammar

bad grammar
Merge pull request #3603 from myqlarson/patch-3

Update rest-quick-start.md: highlight automatic pluralization
minor update [skip ci]
limit applied to migration
Fixes #3450: added note about the fact that $q parameter is mandatory for aggregare query methods
Fixes #3590: added info about passing parameters to layout
Fixed typo
guide WIP [skip ci]
Fix ErrorHandler
Merge pull request #3612 from gonimar/patch-16

Fix ErrorHandler
list of predefined Auth clients updated
Url encoding for the route param added
CHANGELOG.md updated
Merge pull request #3614 from klimov-paul/route-url-encode

Route parameter url encoding added
improved PDF version of the guide

- better title page,
- book style,
- improved internal linking using labels and references