Collective.taskqueue

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- Add support for Plone 5.2
[gforcada]

- Fix to use implementer decorator
[petschki]

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- Fix issue where got bool header value caused error on task creationg
[datakurre]

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- Fix issue where task queue request with method POST created from GET method
failed because of empty payload in the original request
[datakurre]

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- Add support for Plone 5
[datakurre]
- Fix issue where additional params could not be appended for url with query string
[datakurre]

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- Fix problems with conflicting transactions: only enqueue tasks when
transaction is successfully finished.
[jone]

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- Replace NoRollbackSavepoint with rollback 'supporting' DummySavepoint
[datakurre]