Swift logs: send the content-encoding header if we know it

This should cause an "index.html.gz" file to be transmitted with:
  Content-Type: index/html
  Content-Encoding: gzip

Change-Id: Ifae88b03e048a909b644163526de2fc43e4815bd

roles/wip-upload-logs-swift/library/zuul_swift_upload.py

@@ -423,6 +423,8 @@ class Uploader(object):
                         headers['content-encoding'] = 'deflate'
                         data = DeflateFilter(open(fd.full_path, 'rb'))
                     else:
+                        if fd.encoding:
+                            headers['content-encoding'] = fd.encoding
                         data = open(fd.full_path, 'rb')
                 else:
                     data = ''