Issue with to_clipboard(excel=True) with negative floats
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import numpy as np
pd.DataFrame({ 'A' : np.arange(-10, 10 + 1 ,1.1)}).to_clipboard(excel=True)

```
#### Problem description

When using to_clipboard(excel=True) with a negative float number, and pasting it on Excel, it shows 
 '#FIELD'!  The reason is that it is adding ‘=’ to all negatives floats.

Example: If value is '-10.0' it will pass '=-10.0'
With positive values or ints it won’t happen



<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.9.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None
pandas           : 0.25.0
numpy            : 1.16.4
pytz             : 2019.1
dateutil         : 2.8.0
pip              : 19.1.1
setuptools       : 41.0.1
Cython           : 0.29.12
pytest           : None
hypothesis       : None
sphinx           : 2.1.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.7.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.0
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.0
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
```

</details>


![Screenshot_1](https://user-images.githubusercontent.com/53531652/62228972-9c58dd00-b3b6-11e9-90a4-3905217e5c2a.png)
