BUG, COMPAT: Fix multi-char sep + non-utf8 in read_csv for Python 2
Title is self-explanatory.  Closes #3404.

Not sure if so many different ones to test is not overkill, but looks good, thanks
 @jorisvandenbossche : it was a good thing you asked about a specific test.  You caught a Python 2 bug.  Turns out when we have a `StringIO` (the Python 2 `BytesIO`) of encoded data, we can't wrap it with a `TextIOWrapper` and pass in the `encoding` parameter like we do in Python 3 so that we can decode it.

Normally, this would in fact be taken care by `CsvReader`, BUT that's only for standard delimiters.  When we have a multi-char separator, we have to resort to an ad-hoc method of creating a generator, which does zero decoding.

In short, replace the `Testing` label with the `Bug` and `Compat` labels.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13812?src=pr) is 85.23% (diff: 50.00%)

> Merging [#13812](https://codecov.io/gh/pydata/pandas/pull/13812?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13812   diff @@
==========================================
  Files           140        140          
  Lines         50420      50422     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42975      42977     +2   
  Misses         7445       7445          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [cc216ad...e085fef](https://codecov.io/gh/pydata/pandas/compare/cc216ada82a2b06a52524da7b1559a7f5e31b1c1...e085fef5fa737b93d2980ff9076df19fe7da815f?src=pr)
 @jorisvandenbossche : Squashed the bug, and Travis is passing <a href="https://travis-ci.org/pydata/pandas/builds/147951933">here</a> (had to make a doc change after, hence the `[ci skip]`).  Ready to merge if there are no concerns.
 thanks @gfyoung 
