Buggy handling of "" or " " values for SelectBone

[phorward]

#1361 wanted to introduce this RelSkel as part of a RecordBone:

class FilterRow(RelSkel):
    name = StringBone(
        required=True,
    )
    op = SelectBone(
        required=True,
        values={
            "": "=",
            "$lt": "<",
            "$gt": ">",
            "$lk": "like",
        },
        defaultValue=" ",
    )
    value = StringBone(
        required=True,
    )
    
filters = RecordBone(
    descr="Filter",
    using=FilterRow,
    multiple=True,
    format="$(name)$(op)=$(value)"
)

The SelectBone neither is able to process a value "", nor " ".

To my amazement, a " " is turned into a "_" when processed by ViUR, it seems that this is part of the gunicorn-stuff already, but I have to investigate.

Something is wrong here.