fixing pandas.DataFrame.plot(): labels do not appear in legend and label kwd
Closes https://github.com/pydata/pandas/issues/9542, #8905

The following behavior has been tested: 
- `df.plot(y='sin(x)')` -> gives a legend with label 'None' -> this should give no legend instead (as it plots one series, and then we don't automatically add a legend, see behaviour of df['sin(x)'].plot())
- `df.plot(y='sin(x)', legend=True)` -> gives a legend with label 'None' -> this should of course give a legend with label 'sin(x)' (behaviour as df['sin(x)'].plot(legend=True))
- `df.plot(y='sin(x)', label='something else', legend=True)` -> gives a legend with label 'None' -> should be a legend with label 'something else', as we want that the label kwarg overwrites the column name.

based on following data:
`x=np.linspace(-10,10,201)`
`y,z=np.sin(x),np.cos(x)`
`x,y,z=pd.Series(x),pd.Series(y),pd.Series(z)`
`df=pd.concat([x,y,z],axis=1)`
`df.columns=['x','sin(x)','cos(x)']`
`df=df.set_index('x')`
