Default to ENV['RABBITMQ_URL'] when no connection argument is given

- Bunny.new and Bunny.run now defaults to ENV['RABBITMQ_URL'] for connection
  argument.
- Session.new now only uses ENV['RABBITMQ_URL'] for connection argument
  when no other argument is given.

lib/bunny.rb

@@ -58,7 +58,7 @@ def self.protocol_version
   # @see http://rubybunny.info/articles/getting_started.html
   # @see http://rubybunny.info/articles/connecting.html
   # @api public
-  def self.new(connection_string_or_opts = {}, opts = {}, &block)
+  def self.new(connection_string_or_opts = ENV['RABBITMQ_URL'], opts = {}, &block)
     if connection_string_or_opts.respond_to?(:keys) && opts.empty?
       opts = connection_string_or_opts
     end
@@ -70,7 +70,7 @@ def self.new(connection_string_or_opts = {}, opts = {}, &block)
   end
 
 
-  def self.run(connection_string_or_opts = {}, opts = {}, &block)
+  def self.run(connection_string_or_opts = ENV['RABBITMQ_URL'], opts = {}, &block)
     raise ArgumentError, 'Bunny#run requires a block' unless block
 
     client = Session.new(connection_string_or_opts, opts)

lib/bunny/session.rb

@@ -125,12 +125,12 @@ class Session
     # @see http://rubybunny.info/articles/connecting.html Connecting to RabbitMQ guide
     # @see http://rubybunny.info/articles/tls.html TLS/SSL guide
     # @api public
-    def initialize(connection_string_or_opts = Hash.new, optz = Hash.new)
-      opts = case (ENV["RABBITMQ_URL"] || connection_string_or_opts)
+    def initialize(connection_string_or_opts = ENV['RABBITMQ_URL'], optz = Hash.new)
+      opts = case (connection_string_or_opts)
              when nil then
                Hash.new
              when String then
-               self.class.parse_uri(ENV["RABBITMQ_URL"] || connection_string_or_opts)
+               self.class.parse_uri(connection_string_or_opts)
              when Hash then
                connection_string_or_opts
              end.merge(optz)