Inserting code blocks is broken

[zickgraf]

#218 breaks inserting code blocks.

Steps to reproduce:

1. Replace Chunk by Code in tst/worksheets/general.sheet/worksheet.g
2. Run tests

Expected result: Hello, world appears in tst/worksheets/general.actual/_Chunks.xml.

Actual result: Hello, world does not appear in tst/worksheets/general.actual/_Chunks.xml.

Real life example: https://github.com/homalg-project/CAP_project/blob/master/CAP/doc/Limits.autodoc#L40