df.melt() overwriting column named 'variable'
when melting df using a column named 'variable' as an id_var.
resultant frame has original 'variable' column overwritten with the newly melted 'variable' column.
would suggest checking for presence of 'variable' column and appending '_i' to newly created columns.

[example_melt_overwrite.zip](https://github.com/pandas-dev/pandas/files/5510886/example_melt_overwrite.zip)

foo = pd.read_pickle('example_melt_overwrite.pkl')
foo.melt(id_vars=['DateTime','variable'])
