Be more defensive: handle authentication mechanism lists that are blank

lib/bunny/session.rb

@@ -191,7 +191,7 @@ def initialize(connection_string_or_opts = ENV['RABBITMQ_URL'], optz = Hash.new)
 
       client_props         = opts[:properties] || opts[:client_properties] || {}
       @client_properties   = DEFAULT_CLIENT_PROPERTIES.merge(client_props)
-      @mechanism           = opts.fetch(:auth_mechanism, "PLAIN")
+      @mechanism           = normalize_auth_mechanism(opts.fetch(:auth_mechanism, "PLAIN"))
       @credentials_encoder = credentials_encoder_for(@mechanism)
       @locale              = @opts.fetch(:locale, DEFAULT_LOCALE)
 
@@ -1394,6 +1394,17 @@ def normalize_client_channel_max(n)
       end
     end
 
+    def normalize_auth_mechanism(value)
+      case value
+      when [] then
+        "PLAIN"
+      when nil then
+        "PLAIN"
+      else
+        value
+      end
+    end
+
     def ignoring_io_errors(&block)
       begin
         block.call