BF: Set headers only if response not sent
BF: Set headers only if response not sent

Prevents headers being sent multiple times due to multiple callbacks
being set from REST clients. This fixes #203.

truststamp/S3,lucisilv/Silviu_S3-Antidote,scality/S3,lucisilv/Silviu_S3-Antidote,truststamp/S3,scality/S3,truststamp/S3,tomateos/S3,truststamp/S3,tomateos/S3,tomateos/S3,scality/S3,tomateos/S3,lucisilv/Silviu_S3-Antidote,tomateos/S3,tomateos/S3,scality/S3,scality/S3,truststamp/S3,lucisilv/Silviu_S3-Antidote,scality/S3,scality/S3,lucisilv/Silviu_S3-Antidote,scality/S3,tomateos/S3,truststamp/S3,lucisilv/Silviu_S3-Antidote,truststamp/S3,lucisilv/Silviu_S3-Antidote,tomateos/S3,truststamp/S3,lucisilv/Silviu_S3-Antidote