conventions: decode unsigned integers to signed if _Unsigned=false #4966

netCDF3 doesn't know unsigned while OPeNDAP doesn't know signed (bytes). Depending on which backend source is used, the original data is stored with the wrong signedness and needs to be decoded based on the _Unsigned attribute. While the netCDF3 variant is already implemented, this commit adds the symmetric case covering OPeNDAP. closes pydata#4954