COSWriter.doWriteHeader(COSDocument) should use the document specified as parameter

COSWriter.doWriteHeader(COSDocument) currently takes a COSDocument as a parameter and its javadoc specifies that this document is used to retrieve the header data from. Still, the implementation actually uses the property fdDocument or pdDocument to get the pdf version number. This is usually equivalent to using the passed document, but there may be cases where this violates the contract specified in the javadoc. Besides, using the passed doc results in slightly less verbose code.