Clear download errors on reassigning remote urls

Closes #2725

lib/carrierwave/mounter.rb

@@ -125,6 +125,7 @@ def remote_urls=(urls)
         urls = Array.wrap(urls).reject(&:blank?)
         return if urls.blank?
       end
+      @download_errors.clear
       @remote_urls = urls
 
       clear_unstaged

spec/mount_multiple_spec.rb

@@ -881,6 +881,15 @@ def monkey
 
         it { is_expected.to include(a_kind_of(CarrierWave::DownloadError)) }
       end
+
+      context "on the second attempt" do
+        it "clears the existing download errors" do
+          instance.remote_images_urls = ["http://www.example.com/missing.jpg"]
+          is_expected.not_to be_empty
+          instance.remote_images_urls = ["http://www.example.com/#{test_file_name}"]
+          is_expected.to be_empty
+        end
+      end
     end
 
     describe '#write_images_identifier' do

spec/mount_single_spec.rb

@@ -586,6 +586,13 @@ def monkey
         @instance.remote_image_url = "http://www.example.com/missing.jpg"
         expect(@instance.image_download_error).to be_an_instance_of(CarrierWave::DownloadError)
       end
+
+      it "clears the existing download error on the second attempt" do
+        @instance.remote_image_url = "http://www.example.com/missing.jpg"
+        expect(@instance.image_download_error).to be_an_instance_of(CarrierWave::DownloadError)
+        @instance.remote_image_url = "http://www.example.com/test.jpg"
+        expect(@instance.image_download_error).to be_nil
+      end
     end
 
     describe '#image_download_error' do

spec/orm/activerecord_spec.rb

@@ -557,6 +557,7 @@ def monkey
     describe "#remote_image_url=" do
       before do
         stub_request(:get, "http://www.example.com/test.jpg").to_return(body: File.read(file_path("test.jpg")))
+        stub_request(:get, "http://www.example.com/missing.jpg").to_return(status: 404)
       end
 
       # FIXME: ideally image_changed? and remote_image_url_changed? would return true
@@ -569,6 +570,15 @@ def monkey
         expect(@event.image_changed?).to be_falsey
       end
 
+      it "should clear validation errors from the previous attempt" do
+        @event.remote_image_url = 'http://www.example.com/missing.jpg'
+        expect(@event).to_not be_valid
+        expect(@event.errors[:image]).to eq(['could not download file: 404 ""'])
+        @event.remote_image_url = 'http://www.example.com/test.jpg'
+        expect(@event).to be_valid
+        expect(@event.errors).to be_empty
+      end
+
       context 'when validating download' do
         before do
           @uploader.class_eval do
@@ -1570,6 +1580,7 @@ def monkey
     describe "#remote_images_urls=" do
       before do
         stub_request(:get, "http://www.example.com/test.jpg").to_return(body: File.read(file_path("test.jpg")))
+        stub_request(:get, "http://www.example.com/missing.jpg").to_return(status: 404)
       end
 
       # FIXME: ideally images_changed? and remote_images_urls_changed? would return true
@@ -1582,6 +1593,15 @@ def monkey
         expect(@event.images_changed?).to be_falsey
       end
 
+      it "should clear validation errors from the previous attempt" do
+        @event.remote_images_urls = ['http://www.example.com/missing.jpg']
+        expect(@event).to_not be_valid
+        expect(@event.errors[:images]).to eq(['could not download file: 404 ""'])
+        @event.remote_images_urls = ['http://www.example.com/test.jpg']
+        expect(@event).to be_valid
+        expect(@event.errors).to be_empty
+      end
+
       context 'when validating download' do
         before do
           @uploader.class_eval do