Don't apply Scope's transaction name to the event if it already has one

This makes transaction event's transaction being overriden when
`Transaction#finish` is called.

Fixes #1540.

sentry-ruby/lib/sentry/scope.rb

@@ -23,14 +23,14 @@ def apply_to_event(event, hint = nil)
       event.user = user.merge(event.user)
       event.extra = extra.merge(event.extra)
       event.contexts = contexts.merge(event.contexts)
+      event.transaction ||= transaction_names.last
 
       if span
         event.contexts[:trace] = span.get_trace_context
       end
 
       event.fingerprint = fingerprint
       event.level = level
-      event.transaction = transaction_names.last
       event.breadcrumbs = breadcrumbs
       event.rack_env = rack_env if rack_env
 

sentry-ruby/spec/sentry/transaction_spec.rb

@@ -338,6 +338,15 @@
       expect(event[:spans]).to be_empty
     end
 
+    it "doesn't override the event's transaction attribute with the scope's" do
+      subject.finish
+
+      expect(events.count).to eq(1)
+      event = events.last.to_hash
+
+      expect(event[:transaction]).to eq("foo")
+    end
+
     describe "hub selection" do
       it "prioritizes the optional hub argument and uses it to submit the transaction" do
         expect(another_hub).to receive(:capture_event)