record dumper assumes content type and content length

[jnioche]

As stated in [http://iipc.github.io/warc-specifications/specifications/warc-format/warc-1.0/#content-type]

All records with a non-empty block (non-zero Content-Length), except ‘continuation’ records, should have a Content-Type field. Only if the media type is not given by a Content-Type field, a reader may attempt to guess the media type via inspection of its content and/or the name extension(s) of the URI used to identify the resource. If the media type remains unknown, the reader should treat it as type “application/octet-stream”.

This is a should not a must. The record dumper should not assume that a record has a content type or content length. It currently crashes on such records but should be able to handle such cases.