HTTP: fixed promise subrequest() with error_page redirect.

HTTP: fixed promise subrequest() with error_page redirect.

Previously, promise callbacks for a subrequest were stored in a
subrequest context.  This is incorrect because a subrequest content may
be destroyed (for example when error_page with redirect is enabled for a
subrequest location).

The fix is to store callbacks in the parent context.

The issue was introduced in 6fccbc9f1288 (0.3.8).