Adding offset to Timestamp discards freq information when freq="M" or "W"
#### Code Sample, a copy-pastable example if possible

```python
ts = pd.Timestamp("2019-01-01", freq="M")
ts1 = ts + 1 * ts.freq
print(ts1.freq)
```

#### Problem description

There seem to be a problem when a new `pd.Timestamp` is constructed by adding a multiple of the frequency to an initial timestamp: when the frequency is `"M"` or `"W"`, then the frequency information is lost in the resulting timestamp.

I'm not sure if this is intended behaviour (for some reason), but it seems to be inconsistent across frequencies.

#### Expected Output

```
<MonthEnd>
```

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]

INSTALLED VERSIONS
------------------
commit: None

pandas: 0.24.2
pytest: None
pip: 19.1.1
setuptools: 41.0.1
Cython: None
numpy: 1.16.4
scipy: None
pyarrow: None
xarray: None
IPython: 7.6.0
sphinx: None
patsy: None
dateutil: 2.8.0
pytz: 2019.1
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
