[ruby/agoo] Use constants for HTTP headers (#8958)

Constants improve performance as these string don't need to be allocated
again after each request.

Also replace `Time.now.utc.httpdate` with `Time.now.httpdate` as is used
by other Ruby frameworks.

frameworks/Ruby/agoo/app.rb

@@ -20,6 +20,17 @@ ALL_IDS = QUERY_RANGE.to_a
 QUERIES_MIN = 1
 QUERIES_MAX = 500
 
+CONTENT_TYPE = 'Content-Type'
+CONTENT_LENGTH = 'Content-Length'
+DATE = 'Date'
+SERVER = 'Server'
+SERVER_STRING = 'Agoo'
+
+JSON_TYPE = 'application/json'
+HTML_TYPE = 'text/html; charset=utf-8'
+PLAINTEXT_TYPE = 'text/plain'
+
+
 class BaseHandler
   def self.extract_queries_param(request = nil)
     queries = Rack::Utils.parse_query(request['QUERY_STRING'])['queries'].to_i rescue 1
@@ -45,9 +56,9 @@ class BaseHandler
     [
         200,
         {
-            'Content-Type' => 'text/html; charset=utf-8',
-            'Date'         => Time.now.utc.httpdate,
-            'Server'       => 'Agoo'
+            CONTENT_TYPE => HTML_TYPE,
+            DATE         => Time.now.httpdate,
+            SERVER       => SERVER_STRING
         },
         [str]
     ]
@@ -57,9 +68,9 @@ class BaseHandler
     [
         200,
         {
-            'Content-Type' => 'application/json',
-            'Date'         => Time.now.utc.httpdate,
-            'Server'       => 'Agoo'
+            CONTENT_TYPE => JSON_TYPE,
+            DATE         => Time.now.httpdate,
+            SERVER       => SERVER_STRING
         },
         [Oj.dump(obj, { :mode => :strict })]
     ]
@@ -69,9 +80,9 @@ class BaseHandler
     [
         200,
         {
-            'Content-Type' => 'text/plain',
-            'Date'         => Time.now.utc.httpdate,
-            'Server'       => 'Agoo'
+            CONTENT_TYPE => PLAINTEXT_TYPE,
+            DATE         => Time.now.httpdate,
+            SERVER       => SERVER_STRING
         },
         [str]
     ]