pd.concat removes column names when not using index
If I merge two data frames by columns ignoring the indexes, it seems the column names get lost on the resulting object, being replaced instead by integers. Example:
```python
import numpy as np, pandas as pd

np.random.seed(1)
df1 = pd.DataFrame(np.random.random(size = (10,5)), columns = ['col1', 'col2', 'col3', 'col4', 'col5'])
df2 = pd.DataFrame(np.random.poisson(5, size = (10, 3)), columns = ['cl1', 'cl2', 'cl3'])
df2 = pd.concat([df2, df1], axis = 1, ignore_index = True)
df2.columns
```
Returns:
```RangeIndex(start=0, stop=8, step=1)```

When using `ignore_index = False` however, the column names remain in the merged object:
```python
import numpy as np, pandas as pd

np.random.seed(1)
df1 = pd.DataFrame(np.random.random(size = (10,5)), columns = ['col1', 'col2', 'col3', 'col4', 'col5'])
df2 = pd.DataFrame(np.random.poisson(5, size = (10, 3)), columns = ['cl1', 'cl2', 'cl3'])
df2 = pd.concat([df2, df1], axis = 1, ignore_index = False)
df2.columns
```
Returns:
```Index(['cl1', 'cl2', 'cl3', 'col1', 'col2', 'col3', 'col4', 'col5'], dtype='object')```