fix(rng export): remove rng:data if element have fixed_value

Before this patch, the test was done on the last child.
After this patch, there is a search on all children.

This is needed for CustodialHistoryItem


Fix #15

--HG--
branch : seda21
4 jobs for !14 with topic/seda21/cutodial-history-item in 1 minute and 43 seconds (queued for 23 seconds)
latest detached
Status Job ID Name Coverage
  Lint
passed check-manifest #310542

00:00:27

passed flake8 #310541

00:00:42

 
  Tests
failed py27 #310543

00:01:01

failed py3 #310544

00:00:55

 
Name Stage Failure
failed
py3 Tests
Uploading artifacts...
WARNING: py3.html: no matching files
WARNING: py3-deprecated-warnings.json: no matching files
ERROR: No files to upload
Uploading artifacts...
WARNING: report.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1
failed
py27 Tests
pytest.py: error: unrecognized arguments: --html=py3.html --self-contained-html
inifile: /builds/cubicweb/cubes/seda/tox.ini
rootdir: /builds/cubicweb/cubes/seda

ERROR: InvocationError for command /builds/cubicweb/cubes/seda/.tox/py27/bin/python -m pytest --html=py3.html --self-contained-html (exited with code 4)
___________________________________ summary ____________________________________
ERROR: py27: commands failed
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1