Stop field snippets requiring a type for missing, optional, ignored field

Missing optional fields require a type to be explicitly provided as
it cannot be inferred from the payload. Ignored fields are not
included in the documentation, yet, previously, a field that was
missing, optional, and ignored would still require a type to be
provided otherwise the test would fail. This was pointless as the
field wasn't going to appear in the documentation.

This commit updates the fields snippets so that a type is no longer
required for a field that is missing, optional and ignored.

Closes gh-289

spring-restdocs-core/src/main/java/org/springframework/restdocs/payload/AbstractFieldsSnippet.java

@@ -100,15 +100,17 @@ public abstract class AbstractFieldsSnippet extends TemplatedSnippet {
 		validateFieldDocumentation(contentHandler);
 
 		for (FieldDescriptor descriptor : this.fieldDescriptors) {
-			try {
-				descriptor.type(contentHandler.determineFieldType(descriptor));
-			}
-			catch (FieldDoesNotExistException ex) {
-				String message = "Cannot determine the type of the field '"
-						+ descriptor.getPath() + "' as it is not present in the "
-						+ "payload. Please provide a type using "
-						+ "FieldDescriptor.type(Object type).";
-				throw new FieldTypeRequiredException(message);
+			if (!descriptor.isIgnored()) {
+				try {
+					descriptor.type(contentHandler.determineFieldType(descriptor));
+				}
+				catch (FieldDoesNotExistException ex) {
+					String message = "Cannot determine the type of the field '"
+							+ descriptor.getPath() + "' as it is not present in the "
+							+ "payload. Please provide a type using "
+							+ "FieldDescriptor.type(Object type).";
+					throw new FieldTypeRequiredException(message);
+				}
 			}
 		}
 

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

@@ -118,6 +118,21 @@ public class RequestFieldsSnippetTests extends AbstractSnippetTests {
 								.request("http://localhost").content("{}").build());
 	}
 
+	@Test
+	public void missingIgnoredOptionalRequestFieldDoesNotRequireAType()
+			throws IOException {
+		this.snippet
+				.expectRequestFields(
+						"missing-ignored-optional-request-field-does-not-require-a-type")
+				.withContents(tableWithHeader("Path", "Type", "Description"));
+		new RequestFieldsSnippet(Arrays
+				.asList(fieldWithPath("a.b").description("one").ignored().optional()))
+						.document(operationBuilder(
+								"missing-ignored-optional-request-field-does-not-require-a-type")
+										.request("http://localhost").content("{}")
+										.build());
+	}
+
 	@Test
 	public void presentOptionalRequestField() throws IOException {
 		this.snippet.expectRequestFields("present-optional-request-field")

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

@@ -147,6 +147,20 @@ public class ResponseFieldsSnippetTests extends AbstractSnippetTests {
 								.response().content("{}").build());
 	}
 
+	@Test
+	public void missingIgnoredOptionalResponseFieldDoesNotRequireAType()
+			throws IOException {
+		this.snippet
+				.expectResponseFields(
+						"missing-ignored-optional-response-field-does-not-require-a-type")
+				.withContents(tableWithHeader("Path", "Type", "Description"));
+		new ResponseFieldsSnippet(Arrays
+				.asList(fieldWithPath("a.b").description("one").ignored().optional()))
+						.document(operationBuilder(
+								"missing-ignored-optional-response-field-does-not-require-a-type")
+										.response().content("{}").build());
+	}
+
 	@Test
 	public void presentOptionalResponseField() throws IOException {
 		this.snippet.expectResponseFields("present-optional-response-field")