Correct type declaration for events

Normal events shouldn't have the `type` field. Only transactions should
have `type: "transaction"`.

But in the logs, we should still print normal events' type as `"event"`.

For more information about the rules, please see https://github.com/getsentry/sentry-ruby/pull/1336/files#r596851969

Author: st0012

sentry-rails/spec/sentry/send_event_job_spec.rb

@@ -56,7 +56,7 @@
 
       expect(transport.events.count).to eq(1)
       event = transport.events.first
-      expect(event.type).to eq("event")
+      expect(event.type).to eq(nil)
     end
 
     context "when ApplicationJob is not defined" do

sentry-ruby/lib/sentry/client.rb

@@ -72,7 +72,7 @@ def event_from_transaction(transaction)
     def send_event(event, hint = nil)
       event_type = event.is_a?(Event) ? event.type : event["type"]
 
-      if event_type == "event" && configuration.before_send
+      if event_type != TransactionEvent::TYPE && configuration.before_send
         event = configuration.before_send.call(event, hint)
 
         if event.nil?
@@ -85,8 +85,9 @@ def send_event(event, hint = nil)
 
       event
     rescue => e
-      logger.error(LOGGER_PROGNAME) { "#{event_type.capitalize} sending failed: #{e.message}" }
-      logger.error(LOGGER_PROGNAME) { "Unreported #{event_type.capitalize}: #{Event.get_log_message(event.to_hash)}" }
+      loggable_event_type = (event_type || "event").capitalize
+      logger.error(LOGGER_PROGNAME) { "#{loggable_event_type} sending failed: #{e.message}" }
+      logger.error(LOGGER_PROGNAME) { "Unreported #{loggable_event_type}: #{Event.get_log_message(event.to_hash)}" }
       raise
     end
 
@@ -110,8 +111,8 @@ def dispatch_async_event(async_block, event, hint)
         async_block.call(event_hash)
       end
     rescue => e
-      event_type = event_hash["type"]
-      logger.error(LOGGER_PROGNAME) { "Async #{event_type} sending failed: #{e.message}" }
+      loggable_event_type = event_hash["type"] || "event"
+      logger.error(LOGGER_PROGNAME) { "Async #{loggable_event_type} sending failed: #{e.message}" }
       send_event(event, hint)
     end
 

sentry-ruby/lib/sentry/event.rb

@@ -100,7 +100,6 @@ def rack_env=(env)
     end
 
     def type
-      "event"
     end
 
     def to_hash

sentry-ruby/lib/sentry/transaction_event.rb

@@ -2,6 +2,8 @@
 
 module Sentry
   class TransactionEvent < Event
+    TYPE = "transaction"
+
     ATTRIBUTES = %i(
       event_id level timestamp start_timestamp
       release environment server_name modules
@@ -17,7 +19,7 @@ def start_timestamp=(time)
     end
 
     def type
-      "transaction"
+      TYPE
     end
 
     def to_hash