Coerce More-Logs-Offset HTTP header value to a string

The Rack spec (https://github.com/rack/rack/blob/master/SPEC.rdoc#the-headers-) expects all HTTP header values to be strings; integers are not allowed. Some Rack servers, such as Thin, fail if you attempt to assign a integer because they attempt to run string operations (e.g. chomp) on the value (https://github.com/macournoyer/thin/blob/f83ab677f008afcfabcdfa8930225f6cea88f461/lib/thin/response.rb#L57).

This came out of a discussion in !27746 (comment 345550754).