Reconcile custom SegySpecs used in the roundtrip SEGY -> MDIO -> SEGY tests

[dmitriyrepin]

For the round trip tests we currently import SegY file to MDIO using a custom SegySpec
However, when export MDIO to SegY file we use the standard SegySpec
That is why the round trip test fails when it checks headers.
We need to use the same spec for both.
How to do it is yet TBD.

[tasansal]

closed by #611