BUG: DataFrame.melt gives unexpected result when column "value" already exists
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Code Sample, a copy-pastable example

```python
df = pd.DataFrame({'col':list('ABC'),
                   'value':range(10,16,2)})
print(df, '\n')

dfm = df.melt(id_vars='value')

print(dfm)

  col  value
0   A     10
1   B     12
2   C     14 

  value variable value
0     A      col     A
1     B      col     B
2     C      col     C
```

#### Problem description

When the column `value` already exists,  and we set it as index with `id_vars` it copies the `value` column of the melted dataframe and does not set the "old" `value` column of the original dataframe as `id_vars`
#### Expected Output
```python
# note change of column name 'val' instead of 'value'
df = pd.DataFrame({'col':list('ABC'),
                   'val':range(10,16,2)})

dfm = df.melt(id_vars='val')

print(dfm)
   val variable value
0   10      col     A
1   12      col     B
2   14      col     C
```
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.3.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.5.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.UTF-8

pandas           : 1.0.3
numpy            : 1.18.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 47.1.1.post20200604
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
</details>
