Openpyxl does not handle localized datetime correctly
*Created originally on Bitbucket by [sebdiem (Sébastien Diemer)](https://bitbucket.org/%7B766baa6a-afc5-4a39-aef1-f292f8ae6838%7D/)*
Openpyxl does not handle localized datetime correctly.
Depending on the order of the calls to `openpyxl.utils.datetime.to_excel`, the user might get different results.
This is caused by the `@lru_cache` decorator used by the `to_excel` function. When called with the same datetime, localized with different timezones, the cache considers both objects equal (since they effectively are in python).
Here is a script reproducing the bug:
```
#!python
import datetime
import pytz
from openpyxl.utils.datetime import to_excel
date_utc = pytz.utc.localize(datetime.datetime(2015, 7, 24))
date_paris = date_utc.astimezone(pytz.timezone('Europe/Paris'))
print 'Paris', to_excel(date_paris)
print 'UTC', to_excel(date_utc)
>>> Paris 42209.0833333
>>> UTC 42209.0833333
```
If a rerun the script, inverting the last two lines I get the following output:
```
#!python
>>> UTC 42209.0
>>> Paris 42209.0
```
issue