ENH: dropna should accept single column as string in subset argument
#### Is your feature request related to a problem?

When giving a string in the `subset` argument to dropna based on a single column, we get an error: 

```
Index(...) must be called with a collection of some kind, 'C' was passed
```

#### Describe the solution you'd like

`DataFrame.dropna` should accept a single column subset as a string just like `DataFrame.drop_duplicates`

#### API breaking implications

None I could think of

```python
df = pd.DataFrame({"A": [1, 1, 3], "B": list("abc"), "C": [4, np.NaN, 5]})
print(df)

   A  B    C
0  1  a  4.0
1  2  b  NaN
2  3  c  5.0

df = df.dropna(subset="C")
print(df)

   A  B    C
0  1  a  4.0
2  3  c  5.0
```
