Improving assumptions about offset parameter in StrposOptimizer.

Signed-off-by: Rack Lin <racklin@gmail.com>
add $context->headersManager->add('kernel/operators');

Signed-off-by: Rack Lin <racklin@gmail.com>
Merge pull request #82 from racklin/bugfix/subpos

Improving assumptions about offset parameter in StrposOptimizer.