ResolvableEvent#resolve(true) should raise on reassignment

This patch fixes the bug that calling #resolve with raise_on_reassign
= true on an already resolved ResolvableEvent does not raise an Exception.

Fixes: #964

Author: hanazuki

lib/concurrent-ruby/concurrent/promises.rb

@@ -892,7 +892,7 @@ def with_default_executor(executor)
       private
 
       def rejected_resolution(raise_on_reassign, state)
-        Concurrent::MultipleAssignmentError.new('Event can be resolved only once') if raise_on_reassign
+        raise Concurrent::MultipleAssignmentError.new('Event can be resolved only once') if raise_on_reassign
         return false
       end
 

spec/concurrent/promises_spec.rb

@@ -560,6 +560,18 @@ def behaves_as_delay(delay, value)
       expect(event.wait(0, true)).to be_truthy
     end
 
+    specify "#resolve(raise_on_reassign = true)" do
+      event = resolvable_event
+      event.resolve
+      expect { event.resolve }.to raise_error(Concurrent::MultipleAssignmentError)
+    end
+
+    specify "#resolve(raise_on_reassign = false)" do
+      event = resolvable_event
+      event.resolve
+      expect(event.resolve(false)).to be_falsey
+    end
+
     specify "reservation" do
       event = resolvable_event
       expect(event.reserve).to be_truthy