Add support for gzip decompressing in Ruby gem

This commit sets the middleware used by Excon connections to include the
defaults and Excon::Middleware::Decompress, which decompresses gzip data
returned from the Mailchimp API.

swagger-config/marketing/ruby/templates/api_client.mustache

@@ -57,7 +57,12 @@ module {{moduleName}}
       headers[:Authorization] = "Bearer #@access_token" if @is_oauth
 
       host = @server.length > 0 ? @host.sub('server', @server) : @host
-      conn = Excon.new(host + path, :headers => headers, :read_timeout => @read_timeout, :write_timeout => @write_timeout, :connect_timeout => @connect_timeout)
+            conn = Excon.new(host + path,
+                       :headers => headers,
+                       :read_timeout => @read_timeout,
+                       :write_timeout => @write_timeout,
+                       :connect_timeout => @connect_timeout,
+                       :middlewares => [Excon.defaults[:middlewares], Excon::Middleware::Decompress].flatten)
 
       res = nil
       case http_method.to_sym.downcase