ZF-2944: File trailer dictionary keys should not be limited
The entries in a PDF document's file trailer dictionary are currently limited to one of the following ten values: Size, Prev, Root, Encrypt, Info, ID, Index, W, XRefStm, DocChecksum
Most of these entry types are described in the current version of the PDF reference. However, for forward-compatibility, this list should not be limited at all. Future versions of the PDF standard may introduce new dictionary keys which our implementation does not recognize. Additionally, other PDF implementations may add keys which do not appear in any version of the standard (see ZF-2603).
Our behavior should be to preserve and ignore any unrecognized keys. Change the Zend_Pdf_Trailer class and subclasses so that no key name validation is performed. Create and use class constants for the trailer keys generated/consumed by the framework to protect against bugs.