regression in 0.24: to_dict sometimes loses column names
#### Code Sample

```python
import pandas as pd
pd.DataFrame([[1,2,3],[4,5,6],[7,8,9]],columns=["a?","b!","c"]).to_dict(orient="records")
```

#### Problem description

Column names with non-alphanumeric characters are replaced with `"_n"`  in `0.24.0`:
```python
[{'_0': 1, '_1': 2, 'c': 3}, {'_0': 4, '_1': 5, 'c': 6}, {'_0': 7, '_1': 8, 'c': 9}]
```

#### Expected Output

In `0.23.4`:
```python
[{'a?': 1, 'b!': 2, 'c': 3}, {'a?': 4, 'b!': 5, 'c': 6}, {'a?': 7, 'b!': 8, 'c': 9}]
```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.2.final.0
python-bits: 64
OS: Darwin
OS-release: 17.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: C
LOCALE: en_US.UTF-8

pandas: 0.24.0
pytest: None
pip: 19.0.1
setuptools: 40.7.0
Cython: None
numpy: 1.16.0
scipy: 1.2.0
pyarrow: None
xarray: None
IPython: 7.2.0
sphinx: None
patsy: None
dateutil: 2.7.5
pytz: 2018.9
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.2
openpyxl: None
xlrd: 1.2.0
xlwt: None
xlsxwriter: 1.1.2
lxml.etree: 4.3.0
bs4: None
html5lib: None
sqlalchemy: 1.2.17
pymysql: None
psycopg2: 2.7.7 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None
</details>
