"npmat @ pdseries" should be equivalent to "npmat @ pdseries.values"
#### Code Sample

```python
import pandas as pd
import numpy as np

npmat = np.array([[1, 0],[1, 1]])
pdseries = pd.Series([1, 2])

np.testing.assert_allclose(npmat @ pdseries, npmat @ pdseries.values ) # fails but should succeed

```
#### Problem description

Maybe there is a good explanation but I did not expect this.
It is unexpected that ```npmat @ pdseries``` is NOT equivalent to ```npmat @ pdseries.values```.
It is unexpected that ```npmat @ pdseries```, is equvivalent  to ```pdseries.values @ npmat```. 
 
#### Expected Output
***nothing***
#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.23.4
pytest: 3.7.1
pip: 18.1
setuptools: 36.5.0.post20170921
Cython: 0.26.1
numpy: 1.14.5
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.0
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
