Handle HTTP headers per RFC 8187

According to RFC 8187, HTTP headers should use 7-bit ASCII encoding.
The glanceclient was encoding them as UTF-8, which can leave the 8th
bit nonzero when representing unicode, and which presents problems
for any recipient following the standard and decoding the headers as
ASCII.

This change requires keystoneauth1 3.6.2, which has a fix for a
bug that made it unable to handle bytes in headers.  The dependency
is a patch bumping the keystoneauth1 version in upper-constraints.

Depends-on: https://review.openstack.org/#/c/569138/

Change-Id: I0d14974126fcb20e23a37347f4f1756c323cf2f5
Closes-bug: #1766235

requirements.txt

@@ -3,7 +3,7 @@
 # process, which may cause wedges in the gate later.
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 PrettyTable<0.8,>=0.7.1 # BSD
-keystoneauth1>=3.4.0 # Apache-2.0
+keystoneauth1>=3.6.2 # Apache-2.0
 requests>=2.14.2 # Apache-2.0
 warlock<2,>=1.2.0 # Apache-2.0
 six>=1.10.0 # MIT