df.to_clipboard() creates wrong output for Excel
Copy pasting clipboard values to excel arbitrarily (?) fills nans with 0s.

### Steps
```python
from pandas import Timestamp
from numpy import nan

data = {'asset': {Timestamp('2010-06-29 00:00:00'): nan,
  Timestamp('2010-06-30 00:00:00'): -0.002511469339115502,
  Timestamp('2010-07-01 00:00:00'): -0.07847255560218203,
  Timestamp('2010-07-02 00:00:00'): -0.12568297475787682,
  Timestamp('2010-07-06 00:00:00'): -0.160937491617839,
  Timestamp('2010-07-07 00:00:00'): -0.019242767272329764,
  Timestamp('2010-07-08 00:00:00'): 0.10506322784810118,
  Timestamp('2010-07-09 00:00:00'): -0.0034363690398837576,
  Timestamp('2010-07-12 00:00:00'): -0.020114999999999883,
  Timestamp('2010-07-13 00:00:00'): 0.06392962251786649},
 'strat_long': {Timestamp('2010-06-29 00:00:00'): nan,
  Timestamp('2010-06-30 00:00:00'): nan,
  Timestamp('2010-07-01 00:00:00'): nan,
  Timestamp('2010-07-02 00:00:00'): nan,
  Timestamp('2010-07-06 00:00:00'): nan,
  Timestamp('2010-07-07 00:00:00'): nan,
  Timestamp('2010-07-08 00:00:00'): nan,
  Timestamp('2010-07-09 00:00:00'): nan,
  Timestamp('2010-07-12 00:00:00'): nan,
  Timestamp('2010-07-13 00:00:00'): nan},
 'strat_short': {Timestamp('2010-06-29 00:00:00'): nan,
  Timestamp('2010-06-30 00:00:00'): nan,
  Timestamp('2010-07-01 00:00:00'): nan,
  Timestamp('2010-07-02 00:00:00'): nan,
  Timestamp('2010-07-06 00:00:00'): nan,
  Timestamp('2010-07-07 00:00:00'): nan,
  Timestamp('2010-07-08 00:00:00'): nan,
  Timestamp('2010-07-09 00:00:00'): nan,
  Timestamp('2010-07-12 00:00:00'): nan,
  Timestamp('2010-07-13 00:00:00'): nan},
 'strat': {Timestamp('2010-06-29 00:00:00'): 0.0,
  Timestamp('2010-06-30 00:00:00'): 0.0,
  Timestamp('2010-07-01 00:00:00'): 0.0,
  Timestamp('2010-07-02 00:00:00'): 0.0,
  Timestamp('2010-07-06 00:00:00'): 0.0,
  Timestamp('2010-07-07 00:00:00'): 0.0,
  Timestamp('2010-07-08 00:00:00'): 0.0,
  Timestamp('2010-07-09 00:00:00'): 0.0,
  Timestamp('2010-07-12 00:00:00'): 0.0,
  Timestamp('2010-07-13 00:00:00'): 0.0}}

df = pd.DataFrame.from_dict(data)
```
which creates the following dataset:

<img width="345" alt="Screenshot 2019-12-03 at 11 57 09" src="https://user-images.githubusercontent.com/3731173/70049203-0f23d000-15c4-11ea-9156-7474784a2f1d.png">

Copy pasting clipboard values to excel:

```python
df.to_clipboard()
```

arbitrarily (?) fills nans with 0s:

<img width="415" alt="Screenshot 2019-12-03 at 11 54 32" src="https://user-images.githubusercontent.com/3731173/70049306-51e5a800-15c4-11ea-85a5-e563d66c3687.png">

Passing the `excel=False` flag works (counter-intuitively) as expected:

```
df.to_clipboard(excel=False)
```
<img width="407" alt="Screenshot 2019-12-03 at 11 55 04" src="https://user-images.githubusercontent.com/3731173/70049367-73469400-15c4-11ea-85d2-43c7dc0b3625.png">

### pd.show_versions()
<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.0.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 0.25.3
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 42.0.2
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.1
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.9.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.1.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
```

</details>