[#2847] Correctly encode HTTP to SPDY if X-SPDY-Associated-To-Stream-ID is not present

Motivation:

Because of a bug a NPE was thrown when tried to encode HTTP to SPDY and no X-SPDY-Associated-To-Stream-ID was present.

Modifications:

Use 0 as default value when X-SPDY-Associated-To-Stream-ID not present.

Result:

No NPE anymore.