Merge branch 'release-2.2' into develop

* release-2.2:
  Bump version for xPDO 2.2.6-pl release
Update copyright dates
Bump version for 2.2.7-dev
[#9793] Preserve keys in xPDOObject::getMany()

array_merge was recreating numeric keys in xPDOObject::_getRelatedObjectsByFK()
[#6271] Limit reserved class_key column to inherit="single"

This allows the use of a column called class_key in table objects that are not specified as STI (inherit="single")
Bump version for 2.2.7-pl release
xPDO 2.2.7-pl

Merge branch 'release-2.2'

* release-2.2:
  Bump version for 2.2.7-pl release
  [#6271] Limit reserved class_key column to inherit="single"
  [#9793] Preserve keys in xPDOObject::getMany()
  Bump version for 2.2.7-dev
  Update copyright dates