Convert stream broadcasting to a string

ActionCable does some things behind the scenes that expects these
"broadcasting"s or "channel"s to be strings. However it's not
immediately obvious that the value must be a string. So adding this
conversion ensures things work as expected.

actioncable/lib/action_cable/channel/streams.rb

@@ -72,6 +72,7 @@ module Streams
       # Start streaming from the named <tt>broadcasting</tt> pubsub queue. Optionally, you can pass a <tt>callback</tt> that'll be used
       # instead of the default of just transmitting the updates straight to the subscriber.
       def stream_from(broadcasting, callback = nil)
+        broadcasting = String(broadcasting)
         # Don't send the confirmation until pubsub#subscribe is successful
         defer_subscription_confirmation!
 

actioncable/test/channel/stream_test.rb

@@ -16,6 +16,12 @@ def send_confirmation
     end
   end
 
+  class SymbolChannel < ActionCable::Channel::Base
+    def subscribed
+      stream_from :channel
+    end
+  end
+
   test "streaming start and stop" do
     run_in_eventmachine do
       connection = TestConnection.new
@@ -27,6 +33,17 @@ def send_confirmation
     end
   end
 
+  test "stream from non-string channel" do
+    run_in_eventmachine do
+      connection = TestConnection.new
+      connection.expects(:pubsub).returns mock().tap { |m| m.expects(:subscribe).with("channel", kind_of(Proc), kind_of(Proc)).returns stub_everything(:pubsub) }
+      channel = SymbolChannel.new connection, ""
+
+      connection.expects(:pubsub).returns mock().tap { |m| m.expects(:unsubscribe) }
+      channel.unsubscribe_from_channel
+    end
+  end
+
   test "stream_for" do
     run_in_eventmachine do
       connection = TestConnection.new