TST: test_packers.TestMsgpack checks for minimum structure and extra …
…keys

closes #10732 

I wasn't able to reproduce the odd behavior.  Let's see if this passes on Travis. 

yeh this is not a problem, however I mistakenly did this:

IOW this is a non-existing path and you simply get the btyes of the string back. (which is the defined behavior), though a bit unexpected.

```
In [9]: pd.read_msgpack("pandas/io/tests/data/legacy_msgpack/0.16.2/0.16.2_x86_64_linux_3.4.3")
Out[9]: 
[112,
 97,
 110,
 100,
 97,
 115,
 47,
 105,
 111,
 47,
 116,
 101,
 115,
 116,
 115,
 47,
 100,
 97,
 116,
 97,
 47,
 108,
 101,
 103,
 97,
 99,
 121,
 95,
 109,
```
