Add MultipartBodyBuilder

This commit introduces the MultipartBodyBuilder, a builder for multipart
form bodies.

Issue: SPR-16134

spring-web/src/test/java/org/springframework/http/client/MultipartBodyBuilderTests.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2002-2017 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 org.springframework.http.client;
+
+import org.junit.Test;
+
+import org.springframework.core.io.ClassPathResource;
+import org.springframework.core.io.Resource;
+import org.springframework.http.HttpEntity;
+import org.springframework.http.HttpHeaders;
+import org.springframework.util.LinkedMultiValueMap;
+import org.springframework.util.MultiValueMap;
+
+import static org.junit.Assert.*;
+
+/**
+ * @author Arjen Poutsma
+ */
+public class MultipartBodyBuilderTests {
+
+	@Test
+	public void builder() throws Exception {
+		MultiValueMap<String, String> form = new LinkedMultiValueMap<>();
+		form.add("form field", "form value");
+		Resource logo = new ClassPathResource("/org/springframework/http/converter/logo.jpg");
+		HttpHeaders entityHeaders = new HttpHeaders();
+		entityHeaders.add("foo", "bar");
+		HttpEntity<String> entity = new HttpEntity<>("body", entityHeaders);
+
+		MultipartBodyBuilder builder = new MultipartBodyBuilder();
+		builder.part("key", form).header("foo", "bar");
+		builder.part("logo", logo).header("baz", "qux");
+		builder.part("entity", entity).header("baz", "qux");
+
+		MultiValueMap<String, HttpEntity<?>> result = builder.build();
+
+		assertEquals(3, result.size());
+		assertNotNull(result.getFirst("key"));
+		assertEquals(form, result.getFirst("key").getBody());
+		assertEquals("bar", result.getFirst("key").getHeaders().getFirst("foo"));
+
+		assertNotNull(result.getFirst("logo"));
+		assertEquals(logo, result.getFirst("logo").getBody());
+		assertEquals("qux", result.getFirst("logo").getHeaders().getFirst("baz"));
+
+		assertNotNull(result.getFirst("entity"));
+		assertEquals("body", result.getFirst("entity").getBody());
+		assertEquals("bar", result.getFirst("entity").getHeaders().getFirst("foo"));
+		assertEquals("qux", result.getFirst("entity").getHeaders().getFirst("baz"));
+	}
+
+
+}