Add EmptyCollectionWriter for MultipartFormContentProcessor

.travis.yml

@@ -2,7 +2,7 @@
 language: java
 
 jdk:
-  - oraclejdk8
+  - oraclejdk9
 
 install:
   ./mvnw --settings .settings.xml install -DskipTests=true -Dmaven.javadoc.skip=true -Dgpg.skip -B -V

feign-form/src/main/java/feign/form/MultipartFormContentProcessor.java

@@ -33,6 +33,7 @@
 import feign.codec.Encoder;
 import feign.form.multipart.ByteArrayWriter;
 import feign.form.multipart.DelegateWriter;
+import feign.form.multipart.EmptyCollectionWriter;
 import feign.form.multipart.FormDataWriter;
 import feign.form.multipart.ManyFilesWriter;
 import feign.form.multipart.ManyParametersWriter;
@@ -48,6 +49,7 @@
 /**
  *
  * @author Artem Labazin
+ * @author Darren Foong
  */
 @FieldDefaults(level = PRIVATE, makeFinal = true)
 public class MultipartFormContentProcessor implements ContentProcessor {
@@ -63,6 +65,7 @@ public class MultipartFormContentProcessor implements ContentProcessor {
    */
   public MultipartFormContentProcessor (Encoder delegate) {
     writers = new LinkedList<Writer>();
+    addWriter(new EmptyCollectionWriter());
     addWriter(new ByteArrayWriter());
     addWriter(new FormDataWriter());
     addWriter(new SingleFileWriter());

feign-form/src/main/java/feign/form/multipart/EmptyCollectionWriter.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package feign.form.multipart;
+
+import java.util.Collection;
+
+import feign.codec.EncodeException;
+
+/**
+ *
+ * @author Darren Foong
+ */
+public class EmptyCollectionWriter implements Writer {
+
+  @Override
+  public boolean isApplicable (Object value) {
+    return value instanceof Collection && ((Collection) value).isEmpty();
+  }
+
+  @Override
+  public void write (Output output, String boundary, String key, Object value) throws EncodeException {
+    // no-op
+  }
+}