[bug]read multiple csv files error

- Did you find a bug in datatable, or maybe the bug found you?
Tell us what it is.

- How to reproduce the bug?
**This part is the most important**. Please provide a code snippet which, when
executed in console, demonstrates the problem. If necessary, attach any data
files to the issue.

```python
from datatable import iread, rbind

sel_cols = ['c1', 'c2', 'c3', 'c4']
files = ["/data/" + str(f) + ".csv" for f in df_site['station_id'].tolist()]
dt_data = iread(files, columns=sel_cols, errors='ignore')
# error 1: if add 'columns' params to iread, then output is empty
dt_all = rbind(dt_data, force=True)
df_data = dt_all.to_pandas()
# error 2: if not add 'columns', then it throws error: Cannot rbind column of type str32 to a column of type int32

```

- What was the expected behavior?
In case it is not obvious, please tell us what result should your code
produce.

it must be return a pandas dataframe, or give me some advice how to convert int32 to str32?

- Your environment?
What is your datatable version, python version, and operating system?

centos 7
python 3.8
datatable 1.0

- Tag the issue with `[bug]` or `[segfault]` (depending on whether it crashes
Python or not).

- Thank you for contributing, and sorry for the inconvenience.