Fix problem with named columns in ibm_db driver

Hello,

I found that a simple query like DB["select * from mytable"] would throw this warning:

`sequel/lib/sequel/adapters/ibmdb.rb:151: warning: Column name specified is not valid`

In trying out a few things, it looks like the `field_type` and `field_precision` calls to the IBM_DB driver only work correctly when calling with a column index, but return false when using a column name.  I even tried this with an older 2.5.10 version of the ibm_db driver, and the same problem happened.