MultiScene 'save_animation' fails if "datasets=" isn't provided

[peters77]

Describe the bug
making a multiscene fails if the dataset is not set in mscn.save_animation('......, datasets=['....'])

To Reproduce
my code:

os.environ['XRIT_DECOMPRESS_PATH'] = '/usr/bin/xRITDecompress'
scenes = []
for slot_files in group_files(glob('/msg4_test/H-000-MSG2__-MSG2*'), reader='seviri_l1b_hrit'):
     scenes.append(Scene(reader='seviri_l1b_hrit', filenames=slot_files))

mscn = MultiScene(scenes)
mscn.load(['IR_108'])

mscn.save_animation('/Pictures/test.mp4', fps=1)

Expected behavior
Multiscene should made and saved in to a .mp4 file

Actual results
Traceback (most recent call last):
File "msg4_test_multiscene_1.py", line 53, in
mscn.save_animation('/Pictures/test.mp4', fps=1)
File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/satpy/multiscene.py", line 424, in save_animation
info_datasets = [scn.get(dataset_id) for scn in info_scenes]
File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/satpy/multiscene.py", line 424, in
info_datasets = [scn.get(dataset_id) for scn in info_scenes]
File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/satpy/scene.py", line 621, in get
return self.datasets.get(key, default)
File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/satpy/readers/init.py", line 339, in get
key = self.get_key(key)
File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/satpy/readers/init.py", line 322, in get_key
best=best, **dfilter)
File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/satpy/readers/init.py", line 239, in get_key
"not {}".format(str(type(key))))
ValueError: Expected 'DatasetID', str, or number dict key, not <class 'xarray.core.dataarray.DataArray'>

Environment Info:

• OS: Linux
• SatPy Version: satpy-0.11.2+43

[djhoese]

Good news: I was able to reproduce this with ABI data. I'll see if I can make a fix.