Fixed ngx_http_modsecurity_header_filter() to call next header filter eventually.

Returning of NGX_OK there may lead to request's hang, in particular when HEAD
method is used.
Fixed ngx_http_modsecurity_body_filter() to properly handle filters chain.

Body filter is a wrong place to call ngx_http_next_header_filter() - that
function is intended to be called from header filters.
In case when there were no errors, body filter should call
ngx_http_next_body_filter() eventually.
While here, also removed useless second return.