fix reading XRECORD objects with no duplicate-handling flag

[brettfo]

When reading and XRECORD object, the only possibilities following the 100/AcDbXrecord code pair are either a code 280 which indicates the duplicate record handling flag, or a code 102 which is just regular data.

This moves the code 280 handler higher in the loop to account for the case where a code 102 immediately follows the 100/AcDbXrecord pair which means any subsequent code 280 must belong to the data section and not the duplicate record handling flag.

Fixes #61

FYI @AleksandarDev

[brettfo]

Note: the full fix for reading XRECORD objects comes from an earlier commit (b42cba2) as well as this PR to correct the code 280 handling.

[brettfo]

One more note: I've recently made the change for this library to target .NETStandard1.2 so if you need the old portable-style project you can do the following locally:

git checkout 1c1d20ec73a69febb2ce5b453c096af9921bb127
git cherry-pick b630ab3596203c84cb77904bf717a9cbb1bdde77

The first commit (checkout) is where I merged the autocad-compat branch into master and the second commit is the commit from this PR.