'since pandas supports concat for string columns, do the same for blaze'

def _dshape(self):

    shape = self.lhs.dshape.shape
    if isinstance(self.lhs.schema.measure, Option):
        schema = self.lhs.schema
    elif isinstance(self.rhs.schema.measure, Option):
        schema = self.rhs.schema
    else:
        (_, lhs_encoding) = self.lhs.schema.measure.parameters
        (_, rhs_encoding) = self.rhs.schema.measure.parameters
        assert (lhs_encoding == rhs_encoding)
        schema = DataShape(String(None, lhs_encoding))
    return DataShape(*(shape + (schema,)))
