HTTPCLIENT-2065: Simplify additon of content type parameters in MultipartEntityBuilder

This closes #320

httpclient5/src/main/java/org/apache/hc/client5/http/entity/mime/MultipartEntityBuilder.java

@@ -112,6 +112,17 @@ public class MultipartEntityBuilder {
         this.contentType = contentType;
         return this;
     }
+    /**
+     *  Add parameter to the current {@link ContentType}.
+     *
+     * @param parameter The name-value pair parameter to add to the {@link ContentType}.
+     * @return the {@link MultipartEntityBuilder} instance.
+     * @since 5.2
+     */
+    public MultipartEntityBuilder addParameter(final BasicNameValuePair parameter) {
+        this.contentType = contentType.withParameters(parameter);
+        return this;
+    }
 
     public MultipartEntityBuilder setCharset(final Charset charset) {
         this.charset = charset;

httpclient5/src/test/java/org/apache/hc/client5/http/entity/mime/TestMultipartEntityBuilder.java

@@ -120,6 +120,20 @@ public class TestMultipartEntityBuilder {
                 entity.getContentType());
     }
 
+    @Test
+    public void testMultipartCustomContentTypeUsingAddParameter() {
+        final MultipartEntityBuilder eb = MultipartEntityBuilder.create();
+        eb.setMimeSubtype("related");
+        eb.addParameter(new BasicNameValuePair("boundary", "yada-yada"));
+        eb.addParameter(new BasicNameValuePair("charset", "ascii"));
+        eb.addParameter(new BasicNameValuePair("my", "stuff"));
+        eb.buildEntity();
+        final MultipartFormEntity entity =  eb.buildEntity();
+        Assert.assertNotNull(entity);
+        Assert.assertEquals("multipart/related; boundary=yada-yada; charset=US-ASCII; my=stuff",
+                entity.getContentType());
+    }
+
     @Test
     public void testMultipartWriteTo() throws Exception {
         final String helloWorld = "hello world";