generateDesignOfDefaults does not handle parameters that are invalid R variable names

[mb706]

Discovered by MariaErdmann in mlr issue #1637. If a parameter contains characters that can not be part of a data.frame column name, they are usually replaced by dots. (Most parts of ParamHelpers handle this correctly.) However, some part of generateDesignOfDefaults gets confused somewhere along the way.

> generateDesignOfDefaults(makeParamSet(makeLogicalParam("a-b", default=TRUE)))
Error in as(des[, p], types[p]) : 
  no method or default for coercing “logical” to “NA”