BUG: maybe_convert_objects on soft converttof datetime can raise unexpectedly
#### Code Sample, a copy-pastable example if possible

```python
from pandas import Series, DataFrame
from dateutil.parser import parse

def transform(x):
    return Series( {
        'time': parse("22:05 UTC+1"),
        'title': 'remove this "title" to remove the error, or remove timezone'
    } )

DataFrame ( ["stub"] ).apply(transform)
```
#### Problem description
It gives `TypeError: unhashable type: 'tzoffset'`, instead of new DataFrame. To reproduce it, it's important to have time with `tzinfo` AND another key of different (non-datetime) type.

#### Expected Output
```
                                                       0
time                            2018-01-23 22:05:00-01:00
title  remove this "title" to remove the error, or re...
```

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.0-26-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: 9.0.1
setuptools: 38.4.0
Cython: None
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 5.5.0
sphinx: 1.6.6
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.0
openpyxl: 2.4.9
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: None
lxml: 4.1.1
bs4: None
html5lib: 1.0.1
sqlalchemy: 1.1.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
