Use encode() instead of strutils.safe_encode() in fake_client

Patch [1] broke expectation of patch [2] from safe_encode method. Patch
[2] was expecting safe_encode to return an object of type bytes but that
was changed by patch [1]. Somehow python requests package expects bytes
object even in python 3, so just encode the content in fake_client.

[1] https://review.openstack.org/62345
[2] https://review.openstack.org/58808

Change-Id: I5eb433c614d95742c05798f5a90af5e5692508b6
Merge "Use encode() instead of strutils.safe_encode() in fake_client"