Fixed wrongly typed checksum field in EmbeddedFileParamDict

[lSoleyl]

Fix for resolver().get() failing with following error when resolving a Ref<Stream<EmbeddedFile>>, if a CheckSum field is present:

Shared, caused by
  Can't parse field params of struct Option < EmbeddedFileParamDict >, caused by
  Can't parse field checksum of struct Option < Date >, caused by
  Failed parsing date

ChekSum is defined as 16-byte string, so PdfString should be a more fitting type.

[s3bk]

Yea, to be honest I am not sure how that ended up being a date. It just makes no sense. I may have done that by accident when converting lots of dates from string to date.