[CLIENT] Use GZIP constant

Author: estolfo

elasticsearch-transport/lib/elasticsearch/transport/transport/http/faraday.rb

@@ -51,7 +51,7 @@ def perform_request(method, path, params={}, body=nil, headers=nil)
           # @return [Connections::Connection]
           #
           def __build_connection(host, options={}, block=nil)
-            options.merge!(headers: { accept_encoding: 'gzip' }) if use_compression?
+            options.merge!(headers: { accept_encoding: GZIP }) if use_compression?
             client = ::Faraday.new(__full_url(host), options, &block)
             apply_headers(client, options)
             Connections::Connection.new :host => host, :connection => client
@@ -67,7 +67,6 @@ def host_unreachable_exceptions
 
           private
 
-          GZIP = 'gzip'.freeze
           GZIP_FIRST_TWO_BYTES = '1f8b'.freeze
           HEX_STRING_DIRECTIVE = 'H*'.freeze
           RUBY_ENCODING = '1.9'.respond_to?(:force_encoding)