Base64 encode the 'X-Trace-Info' header

Instead of sending the trace information via
a json string formatted header send it as a
base64 encoded string. This is more compliant
with how headers are typically sent (not as a
raw json string).

Change-Id: Ibb93866591c791787560a14d677d2eeb20f07006
Merge "Base64 encode the 'X-Trace-Info' header"