Merging Index on Column Output Behavior
Merging two data frames, the first using the index, and the second using a column,with a left join has an unusual behavior.  Logically, the merged Dataframe should keep the index column, and discard the right Dataframe's join column(a right join logically has the reverse argument).  Here is an illustrative example:

``` python

import pandas as pd
df1 = pd.DataFrame({'a':['bb','zz','dd'],'g':[10,20,30]})
df1.set_index(inplace=True)
df2 = pd.DataFrame({'a1':['aa','bb','zz','qq','tt','dd'],'b':[4,2,7,8,6,3]})

dfj = pd.merge(df1,df2,how='left',left_index=True,right_on='a1')

print(dfj)

    g  a1  b
1  10  bb  2
2  20  zz  7
5  30  dd  3

```

This deletes the index column, and outputs the right data frame's column.  I am not sure why this would be desirable over 'a' remaining the index, and only column b being added to the outputted data frame.
