unique columns are shown as non-unique after dropping multi-index
#### Code Sample, a copy-pastable example if possible

```python
df1 = pandas.DataFrame({'a':[1,2,3], 'b':[2,3,4]})
df2 = pandas.DataFrame({'c':[1,2,3], 'd':[2,3,4]})
dic = {'a':df1, 'b':df2}
c = pandas.concat(dic,axis=1)
c.columns = c.columns.droplevel()
print(c.columns[~c.columns.is_unique])

```
#### Problem description

After using droplevels on the multiindex the status of unique does not change, leading to unique columns being shown as non unique when checked. 



#### Expected Output

After dropping the first level of the Multiindex, is_unique should show unique columns based on the new (lower dimensional) Index

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.0-41-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
pandas: 0.22.0
pytest: None
pip: 10.0.1
setuptools: 39.1.0
Cython: 0.28.2
numpy: 1.14.3
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.2
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
