Fix StreamChannelException raises. (#52)

Fixes #40

Author: Eugene

lib/stream-chat/channel.rb

@@ -19,7 +19,7 @@ def initialize(client, channel_type, channel_id = nil, custom_data = nil)
     end
 
     def url
-      raise StreamChannelException 'channel does not have an id' if @id.nil?
+      raise StreamChannelException, 'channel does not have an id' if @id.nil?
 
       "channels/#{@channel_type}/#{@id}"
     end
@@ -85,7 +85,7 @@ def update(channel_data, update_message = nil)
     end
 
     def update_partial(set = nil, unset = nil)
-      raise StreamChannelException 'set or unset is needed' if set.nil? && unset.nil?
+      raise StreamChannelException, 'set or unset is needed' if set.nil? && unset.nil?
 
       payload = { set: set, unset: unset }
       @client.patch(url, data: payload)