Fix ignored ids for belongsToMany relations

Fix ids ignored by marshaller for belongsToMany relations.
Merge branch 'patch-4' of https://github.com/SplicePHP/cakephp into 3.0-fix-btm-marshaller
Updating the new code in Marshalelr to take composite keys into consideration
Fixing part of a failing test
Fixed the rest of the failing test
Improving test to show the internal use of union queries in marshaller
Not using union, but OR conditions
Merge pull request #4999 from cakephp/3.0-fix-btm-marshaller

3.0 fix btm marshaller