BUG: read_csv throws UnicodeDecodeError with unicode aliases
Rebased as PR https://github.com/pydata/pandas/pull/14060

---
- [x] closes #13549 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

-read_csv with engine=c throws error when encoding=UTF_16 (anything other than utf-16)
-improved nosetests and moved to in pandas/io/tests/common.py
-passes `pep8radius upstream/master --diff` and  `git diff upstream/master | flake8 --diff`
-put what's new entry in 0.19.0 in accordance with milestone posted on issue
