[close #1327] Fix double port bind in Rails

The "default" configuration puma level is initialized with a "binds" of `tcp://0.0.0.0:9292`. Puma is designed to be able to bind to multiple ports. 

When a `:port` is sent from Rails along with an empty `user_supplied_options` then the port is treated as a "default". This is merged in with the system defaults, and then later converted into a "binds" via calling `config.port` in the `set_host_port_to_config` method. 

The bug comes due to the "level" of the configuration. Since both are being set on the same "level" the `port` call does not over-write the existing binds but instead prepends to the array. We can fix by ensuring that any binds in a given "level" are empty before setting it.

lib/puma/dsl.rb

@@ -118,6 +118,10 @@ def bind(url)
       @options[:binds] << url
     end
 
+    def clear_binds!
+      @options[:binds] = []
+    end
+
     # Define the TCP port to bind to. Use +bind+ for more advanced options.
     #
     def port(port, host=nil)

lib/rack/handler/puma.rb

@@ -84,6 +84,8 @@ def self.valid_options
       end
     private
       def self.set_host_port_to_config(host, port, config)
+        config.clear_binds! if host || port
+
         if host && (host[0,1] == '.' || host[0,1] == '/')
           config.bind "unix://#{host}"
         elsif host && host =~ /^ssl:\/\//

test/test_rack_handler.rb

@@ -153,6 +153,16 @@ def test_config_wins_over_default
     end
   end
 
+  def test_user_port_wins_over_default_when_user_supplied_is_blank
+    user_port = 5001
+    @options[:user_supplied_options] = []
+    @options[:Port] = user_port
+    conf = Rack::Handler::Puma.config(->{}, @options)
+    conf.load
+
+    assert_equal ["tcp://0.0.0.0:#{user_port}"], conf.options[:binds]
+  end
+
   def test_user_port_wins_over_default
     user_port = 5001
     @options[:Port] = user_port