Impossible to combine JoinDocuments and JoinAnswers

[MichelBartels]

Creating a pipeline using JoinDocuments and JoinAnswer, leads to the following error:

File "/home/ubuntu/haystack/haystack/nodes/other/join_answers.py", line 45, in <listcomp>
    reader_results = [inp["answers"] for inp in inputs]
KeyError: 'answers'

The reason for this is that these lines cause the inputs parameter of the first join node to be propagated down the pipeline. This eventually leads to this condition being false when constructing the input for the next join node resulting in that error.