Hack around text problem in unit tests.

Now that payloads are not allowed to be text, some unit tests will cause crashes
on Python 3 because the payload sent by requests gets passed to FakeKey as a
string instead of raw bytes.

I haven't been able to figure out a way around the issue that doesn't get super
messy inside s3/responses.py so I'm just converting the value to bytes using the
system's default encoding.

tox.ini

@@ -2,6 +2,10 @@
 envlist = py27, py36
 
 [testenv]
+setenv =
+    BOTO_CONFIG=/dev/null
+    AWS_SECRET_ACCESS_KEY=foobar_secret
+    AWS_ACCESS_KEY_ID=foobar_key
 deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/requirements-dev.txt