[Hydrator][Hotfix] Fix addStrategy bug

Check for only collection valued association before throwing exception when adding strategy.
Update src/DoctrineModule/Stdlib/Hydrator/DoctrineObject.php
Merge pull request #146 from davidwindell/patch-4

[Hydrator][Hotfix] Fix addStrategy bug
[Hydrator] Check if null values are allowed
Ignore invalid null values
Create OneToOneEntityNotNullable.php
Update tests/DoctrineModuleTest/Stdlib/Hydrator/DoctrineObjectTest.php
Update docs/hydrator.md
Merge pull request #147 from davidwindell/patch-4

[Hydrator] [#139] Check if null values are allowed
Fix doc
Merge pull request #149 from bakura10/master

Fix hydrator doc