fixed prepareQueryElement() "set" strategy issue
Remove unnecessary ternary statement
Test nested collection key queries with set strategy
Refactor DocumentPersister::prepareQueryElement()

Some object calisthenic improvements to appease @guilhermeblanco.
Return a tuple instead of modifying $fieldName param by reference
Refactor targetDocument processing in prepareQueryElement()

More object calisthenic improvements.
Merge pull request #485 from doctrine/fix/set-strategy-defect

fixed prepareQueryElement() "set" strategy issue