Bring back Unicorn HttpServer check

Turns out this is actually necessary in some places

lib/gitlab/runtime.rb

@@ -34,7 +34,7 @@ module Gitlab
 
       # For unicorn, we need to check for actual server instances to avoid false positives.
       def unicorn?
-        !!defined?(::Unicorn)
+        !!(defined?(::Unicorn) && defined?(::Unicorn::HttpServer))
       end
 
       def sidekiq?

spec/lib/gitlab/runtime_spec.rb

@@ -41,9 +41,11 @@ describe Gitlab::Runtime do
 
   context "unicorn" do
     let(:unicorn_type) { Module.new }
+    let(:unicorn_server_type) { Class.new }
 
     before do
       stub_const('::Unicorn', unicorn_type)
+      stub_const('::Unicorn::HttpServer', unicorn_server_type)
     end
 
     it "identifies itself" do