httpd: fix access to context based pages

The contextHandler was bound to last requested page and
was rejecting access to other values. The internal logic
is change to bound only for 'named' contexts.

RELEASE NOTES:
fixed access to active transfers html and txt pages

Ticket: #7975
Acked-by: Albert Rossi
Target: master, 2.7, 2.6
Require-book: no
Require-notes: yes
Merge pull request #175 from kofemann/iss7975-2.7

Iss7975 2.7