BUG: to_period behaves different between Timestamp and DatetimeIndex with timezones
After 935c5d145 `pandas/tests/indexes/datetimes/test_astype.py` fails depending on the timezone:
in fact 
```
TZ=Etc/GMT+1 pytest pandas/tests/indexes/datetimes/test_astype.py -k TestToPeriod
```
is OK while
```
TZ=Etc/GMT-1 pytest pandas/tests/indexes/datetimes/test_astype.py -k TestToPeriod
```
fails.

This can be nailed down to the following code sample.

#### Code Sample

```python
>>> ts = pd.date_range('1/1/2000', '2/1/2000', tz='Etc/GMT-1')
>>> ts.to_period()[0] == ts[0].to_period()
False
>>> ts.to_period()[0], ts[0].to_period()
(Period('1999-12-31', 'D'), Period('2000-01-01', 'D'))
>>> ts = pd.date_range('1/1/2000', '2/1/2000', tz='Etc/GMT+1')
>>> ts.to_period()[0] == ts[0].to_period()
True
>>> ts.to_period()[0], ts[0].to_period()
(Period('2000-01-01', 'D'), Period('2000-01-01', 'D'))
```
#### Problem description

It seems that `ts.to_period()[0] == ts[0].to_period()` depends on tzinfo, true if west of GMT, false if east of GMT. Before 935c5d145 `ts.to_period()[0] == ts[0].to_period()` is always `True` irrespective of tzinfo.

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

<details>

<pre>
INSTALLED VERSIONS
------------------
commit: 935c5d1453938a284805cb7d3c0c3255390997a7
python: 3.7.0.final.0
python-bits: 64
OS: Darwin
OS-release: 17.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.0.dev0+555.g935c5d145
pytest: 3.7.4
pip: 18.0
setuptools: 39.0.1
Cython: 0.28.5
numpy: 1.15.1
scipy: 1.1.0
pyarrow: None
xarray: 0.10.9
IPython: 6.5.0
sphinx: 1.7.9
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.5
blosc: 1.5.1
bottleneck: 1.2.1
tables: None
numexpr: 2.6.8
feather: None
matplotlib: 3.0.0
openpyxl: 2.5.5
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.1.1
lxml: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.12
pymysql: 0.9.2
psycopg2: None
jinja2: 2.10
s3fs: 0.1.6
fastparquet: 0.1.6
pandas_gbq: None
pandas_datareader: None
gcsfs: 0.1.2
</pre>

</details>
