Index changes during pandas.Dataframe.merge()
I have 2 data frames:

```
df1 = pd.DataFrame({'A':[11,2,31,4,51],
                    'B':[1.2,3.14,1.41,1.7,2.2],
                    'C':['e','w','q','q','s']})
df1.set_index('A', inplace=True)
df1
```

A | B | C 
 -- | -- | --
11 | 1.20 | e
2 | 3.14 | w
31 | 1.41 | q
4 | 1.70 | q
51 | 2.20 | s

and

```
df2 = pd.DataFrame({'C':['e','w','q','s'],
                    'AB':[11,1.2,31,5.9],
                    'CAM':['A',1.2,'-',5.9]})
df2.set_index('C', inplace=True)
df2
```


C | AB | CAM
-- | -- | --
e | 11.0 | A
w | 1.2 | 1.2
q | 31.0 | -
s | 5.9 | 5.9

Notice: `'C'` is a column in `df1` and index in `df2`.

Now, merge these 2 data frames as shown next

`df1.merge(df2, left_on='C', right_index=True, how='left')`

Out:

A | B | C | AB | CAM
-- | -- | -- | -- | --
11 | 1.20 | e | 11.0 | A
2 | 3.14 | w | 1.2 | 1.2
31 | 1.41 | q | 31.0 | -
4 | 1.70 | q | 31.0 | -
51 | 2.20 | s | 5.9 | 5.9

Very much what one would expect.

Let's look at another instance, when `'C'` is a column in `df2` and not an index:
`df1.merge(df2.reset_index(), on='C', how='left')`

Out:

 _   | B | C | AB | CAM
-- | -- | -- | -- | --
0 | 1.20 | e | 11.0 | A
1 | 3.14 | w | 1.2 | 1.2
2 | 1.41 | q | 31.0 | -
3 | 1.70 | q | 31.0 | -
4 | 2.20 | s | 5.9 | 5.9

#### Question: Why does the index change?
Isn't it sensical it to remain unchanged?

