[REVIEW] Retain index correctly during merge left_on right_on

[thomcom]

Fixes #1806

In [1]: import cudf
   ...:
   ...: df = cudf.DataFrame()
   ...: df['a'] = [1,2,3,4,5]
   ...: df['b'] = ['a','b','c','d','e']
   ...: df.index=[5,3,4,2,1]
   ...:
   ...: df2 = cudf.DataFrame()
   ...: df2['a2'] = [1,2,3,4,5]
   ...: df2['res'] = ['a','b','c','d','e']
   ...:
   ...: df.merge(df2,left_index=True,right_index=True,how='left').to_pandas()
Out[1]:
   a  b  a2   res
5  1  a  -1  None
3  2  b   4     d
4  3  c   5     e
2  4  d   3     c
1  5  e   2     b