[Bug] Assumes first row is always table header even if it is not

The script:

```python
print(
html2text.html2text(
"""


item aitem b
item citem d


"""
)
)
```

Results to:
```
item a| item b
---|---
item c| item d
```

- Version by `html2text --version`: 2020.1.16
- Test script
- Python version `python --version`: Python 3.10.2