Fixing broken prediction tests

The issue was due to now, BigML, with the new source server config,
says that in source `spam.csv` the field `Message` in `categorical`
instead of `text`. We have to update the source forcing this field as
`text`.
These tests are not completely fixed because, probably due to a bug in
BigML, the `term_analysis` settings of the field are lost. Once BigML
says what happens in this particular case, the tests will be fixed as
needed.
Fixes broken tets due to changes in BigML algorithm