Fix serialization/deserialization of multiple headers with the same value

This pull request fixes two issues on Python 3:

* multiple headers with the same value were being written to cassettes too many times.  If a header had three values, then each of those three values was written nine times.
* when reading a cassette with multiple header values, only one value of the header was being stubbed.

Neither of these are an issue on Python 2.

Fixes #96.