Improve message when fields cannot be documented due to empty body

Closes gh-278

spring-restdocs-core/src/test/java/org/springframework/restdocs/payload/RequestFieldsSnippetFailureTests.java

@@ -99,6 +99,17 @@ public class RequestFieldsSnippetFailureTests {
 								.content("{ \"a\": { \"c\": 5 }}").build());
 	}
 
+	@Test
+	public void attemptToDocumentFieldsWithNoRequestBody() throws IOException {
+		this.thrown.expect(SnippetException.class);
+		this.thrown.expectMessage(
+				equalTo("Cannot document request fields as the request body is empty"));
+		new RequestFieldsSnippet(Arrays.asList(fieldWithPath("a").description("one")))
+				.document(new OperationBuilder("no-request-body",
+						this.snippet.getOutputDirectory()).request("http://localhost")
+								.build());
+	}
+
 	@Test
 	public void undocumentedXmlRequestField() throws IOException {
 		this.thrown.expect(SnippetException.class);

spring-restdocs-core/src/test/java/org/springframework/restdocs/payload/ResponseFieldsSnippetFailureTests.java

@@ -50,6 +50,17 @@ public class ResponseFieldsSnippetFailureTests {
 	@Rule
 	public ExpectedException thrown = ExpectedException.none();
 
+	@Test
+	public void attemptToDocumentFieldsWithNoResponseBody() throws IOException {
+		this.thrown.expect(SnippetException.class);
+		this.thrown.expectMessage(
+				equalTo("Cannot document response fields as the response body is empty"));
+		new ResponseFieldsSnippet(Arrays.asList(fieldWithPath("a").description("one")))
+				.document(new OperationBuilder("no-response-body",
+						this.snippet.getOutputDirectory()).request("http://localhost")
+								.build());
+	}
+
 	@Test
 	public void undocumentedXmlResponseField() throws IOException {
 		this.thrown.expect(SnippetException.class);