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.

app/controllers/projects/refs_controller.rb

@@ -53,7 +53,7 @@ class Projects::RefsController < Projects::ApplicationController
       format.json do
         logs, next_offset = tree_summary.fetch_logs
 
-        response.headers["More-Logs-Offset"] = next_offset if next_offset
+        response.headers["More-Logs-Offset"] = next_offset.to_s if next_offset
 
         render json: logs
       end

spec/controllers/projects/refs_controller_spec.rb

@@ -73,7 +73,7 @@ describe Projects::RefsController do
 
         cache_key = "projects/#{project.id}/logs/#{project.commit.id}/#{path}/25"
         expect(Rails.cache.fetch(cache_key)).to eq(['logs', 50])
-        expect(response.headers['More-Logs-Offset']).to eq(50)
+        expect(response.headers['More-Logs-Offset']).to eq("50")
       end
     end
   end