`HEADER`/`FILE_NAME` is not read

[dsdante]

Steps to reproduce:

1. Export an IFC from a BIM with a name original.ifc. Make sure its HEADER/FILE_NAME is also original.ifc.
2. Rename the file to renamed.ifc.
3. Open it with the xBim library, check the value of ifcStore.Header.FileName.Name.

Expected value: original.ifc
Actual value: renamed.ifc

Likely cause: https://github.com/xBimTeam/XbimEssentials/blob/master/Xbim.IO.MemoryModel/MemoryModelProvider.cs#L133