Allow non ascii characters to be encoded using Base64

[Jack Williamson (jackwilliamson)]

Solves UnicodeEncodeError which occurs whenever a string that is added to the payload's json contains non-ascii characters.

Fixes #194

[Coveralls (coveralls)]

Coverage decreased (-0.2%) to 80.253% when pulling b4ee519 on jackwilliamson:master into 4550310 on snowplow:master.

[Snowplow CLA bot (snowplowcla)]

Confirmed! Jack Williamson (@jackwilliamson) has signed the Individual Contributor License Agreement. Thanks so much

[Jethro Nederhof (jethron)]

The alternate fix is to revert fd2412a which introduced the incorrect behaviour from what was the correct JSON escaping to begin with. See #123

Either option is cleaner than #188 even though it includes tests.

This has been dropping events and throwing exceptions in production applications for quite a while and should probably have been a higher priority and fixed a lot sooner...

[Oguzhan Unlu (oguzhanunlu)]

Thanks for the contribution Jack Williamson (@jackwilliamson) ! It is cherry-picked to the release branch at #224 .