Improve handling of parameters with null values

Closes gh-199

spring-restdocs-core/src/main/java/org/springframework/restdocs/operation/Parameters.java

@@ -22,6 +22,7 @@ import java.util.List;
 import java.util.Map;
 
 import org.springframework.util.LinkedMultiValueMap;
+import org.springframework.util.StringUtils;
 
 /**
  * The parameters received in a request.
@@ -68,6 +69,9 @@ public class Parameters extends LinkedMultiValueMap<String, String> {
 	}
 
 	private static String urlEncodeUTF8(String s) {
+		if (!StringUtils.hasLength(s)) {
+			return "";
+		}
 		try {
 			return URLEncoder.encode(s, "UTF-8");
 		}

spring-restdocs-core/src/test/java/org/springframework/restdocs/operation/ParametersTests.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright 2014-2016 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.restdocs.operation;
+
+import org.junit.Test;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertThat;
+
+/**
+ * Tests for {@link Parameters}.
+ *
+ * @author Andy Wilkinson
+ */
+public class ParametersTests {
+
+	private final Parameters parameters = new Parameters();
+
+	@Test
+	public void queryStringForNoParameters() {
+		assertThat(this.parameters.toQueryString(), is(equalTo("")));
+	}
+
+	@Test
+	public void queryStringForSingleParameter() {
+		this.parameters.add("a", "b");
+		assertThat(this.parameters.toQueryString(), is(equalTo("a=b")));
+	}
+
+	@Test
+	public void queryStringForSingleParameterWithMultipleValues() {
+		this.parameters.add("a", "b");
+		this.parameters.add("a", "c");
+		assertThat(this.parameters.toQueryString(), is(equalTo("a=b&a=c")));
+	}
+
+	@Test
+	public void queryStringForMutipleParameters() {
+		this.parameters.add("a", "alpha");
+		this.parameters.add("b", "bravo");
+		assertThat(this.parameters.toQueryString(), is(equalTo("a=alpha&b=bravo")));
+	}
+
+	@Test
+	public void queryStringForParameterWithEmptyValue() {
+		this.parameters.add("a", "");
+		assertThat(this.parameters.toQueryString(), is(equalTo("a=")));
+	}
+
+	@Test
+	public void queryStringForParameterWithNullValue() {
+		this.parameters.add("a", null);
+		assertThat(this.parameters.toQueryString(), is(equalTo("a=")));
+	}
+
+	@Test
+	public void queryStringForParameterThatRequiresEncoding() {
+		this.parameters.add("a", "alpha&bravo");
+		assertThat(this.parameters.toQueryString(), is(equalTo("a=alpha%26bravo")));
+	}
+
+}