read_fwf colspecs autodetection could be more flexible
See https://github.com/pydata/pandas/pull/4955#issuecomment-25352643

```
id foo 
1  a
1  a
1  a
123a
```

This works with the snippet in the orig #4488 but fails with the merged #4955 due to the last line.  
It's a common example of a fwf file.
