Behavior of pivot table labeling is inconsistent between list and tuple for aggfunc
#### Code Sample, a copy-pastable example if possible

```python
def entropy(x):
    return 1 # dummy func

vamp = pd.DataFrame({
    'vampire':    tuple('nnyyynnn'),
    'shadow':     tuple('?y?n?yy?'),
    'garlic':     tuple('yynnnnny'),
    'complexion': tuple('prraapar'),
    'accent':     tuple('nnnhohho'),
})

display(vamp)

display(vamp.pivot_table(values='vampire', index='shadow', aggfunc=(''.join, entropy)))

display(vamp.pivot_table(values='vampire', index='shadow', aggfunc=[''.join, entropy]))
```
#### Problem description
The current behavior has different outputs for tuples and lists. This is undocumented and weird. Shouldn't iterables have the same output?

Expected output is that output is consistent. 

[this should explain **why** the current behaviour is a problem and why the expected output is a better solution.]


#### Expected Output

##### Output of tuple
![image](https://user-images.githubusercontent.com/1774409/36803032-1ee86250-1c7c-11e8-8d10-e67e4e810b5e.png)

##### Output of list

![image](https://user-images.githubusercontent.com/1774409/36803059-29cfe4cc-1c7c-11e8-8e08-cee06b4935d2.png)

Expected output is that these 2 match. 

#### Output of ``pd.show_versions()``

<details>
```
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.96+
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 36.2.7
Cython: None
numpy: 1.14.0
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 5.5.0
sphinx: None
patsy: 0.5.0
dateutil: 2.5.3
pytz: 2016.7
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
fastparquet: None
pandas_gbq: 0.3.0
pandas_datareader: None
```
</details>
