HFileDataBlockEncoderImpl uses wrong header size when reading HFiles with no checksums

When reading a .92 HFile without checksums, encoding it, and storing in the block cache, the HFileDataBlockEncoderImpl always allocates a dummy header appropriate for checksums even though there are none. This corrupts the byte[].

Attaching a patch that allocates a DUMMY_HEADER_NO_CHECKSUM in that case which I think is the desired behavior.