Improve handling of null fields beneath an array

Previously, if an array contained objects where a field was sometimes
null and sometimes had a value of a consistent type, the type inferred
type was varies. Furthermore, it was not possible for the user to
specify a type other than varies as a mismatch would be detected.

This commit updates JsonFieldTypeResolver so that, when dealing with
an imprecise field path (i.e. a path for a field within an array),
null values that don't match the common type for the field are
ignored. This produces the following behavior when nulls are involved:

- All null fields results in the null type
- A mixture of nulls and a particular type results in the particular
  type
- A mixture of nulls and two or more other types results in the varies
  type

Closes gh-398

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

@@ -38,7 +38,7 @@ class JsonFieldTypeResolver {
 				if (commonType == null) {
 					commonType = fieldType;
 				}
-				else if (fieldType != commonType) {
+				else if (fieldType != commonType && fieldType != JsonFieldType.NULL) {
 					return JsonFieldType.VARIES;
 				}
 			}

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

@@ -112,13 +112,73 @@ public class JsonFieldTypeResolverTests {
 	}
 
 	@Test
-	public void nonExistentFieldProducesIllegalArgumentException() throws IOException {
+	public void multipleFieldsWithDifferentTypesAndSometimesAbsent() throws IOException {
+		assertThat(
+				this.fieldTypeResolver.resolveFieldType("a[].id",
+						createPayload("{\"a\":[{\"id\":1},{\"id\":true}, { }]}")),
+				equalTo(JsonFieldType.VARIES));
+	}
+
+	@Test
+	public void multipleFieldsWhenSometimesAbsent() throws IOException {
+		assertThat(
+				this.fieldTypeResolver.resolveFieldType("a[].id",
+						createPayload("{\"a\":[{\"id\":1},{ }]}")),
+				equalTo(JsonFieldType.NUMBER));
+	}
+
+	@Test
+	public void multipleFieldsWithDifferentTypesAndSometimesNull() throws IOException {
+		assertThat(
+				this.fieldTypeResolver.resolveFieldType("a[].id",
+						createPayload(
+								"{\"a\":[{\"id\":1},{\"id\":true}, {\"id\":null}]}")),
+				equalTo(JsonFieldType.VARIES));
+	}
+
+	@Test
+	public void multipleFieldsWhenSometimesNull() throws IOException {
+		assertThat(
+				this.fieldTypeResolver.resolveFieldType("a[].id",
+						createPayload("{\"a\":[{\"id\":1},{\"id\":null}]}")),
+				equalTo(JsonFieldType.NUMBER));
+	}
+
+	@Test
+	public void multipleFieldsWhenEitherNullOrAbsent() throws IOException {
+		assertThat(
+				this.fieldTypeResolver.resolveFieldType("a[].id",
+						createPayload("{\"a\":[{},{\"id\":null}]}")),
+				equalTo(JsonFieldType.NULL));
+	}
+
+	@Test
+	public void multipleFieldsThatAreAllNull() throws IOException {
+		assertThat(
+				this.fieldTypeResolver.resolveFieldType("a[].id",
+						createPayload("{\"a\":[{\"id\":null},{\"id\":null}]}")),
+				equalTo(JsonFieldType.NULL));
+	}
+
+	@Test
+	public void nonExistentSingleFieldProducesFieldDoesNotExistException()
+			throws IOException {
 		this.thrownException.expect(FieldDoesNotExistException.class);
 		this.thrownException.expectMessage(
 				"The payload does not contain a field with the path 'a.b'");
 		this.fieldTypeResolver.resolveFieldType("a.b", createPayload("{\"a\":{}}"));
 	}
 
+	@Test
+	public void nonExistentMultipleFieldsProducesFieldDoesNotExistException()
+			throws IOException {
+		this.thrownException.expect(FieldDoesNotExistException.class);
+		this.thrownException.expectMessage(
+				"The payload does not contain a field with the path 'a[].b'");
+		this.fieldTypeResolver.resolveFieldType("a[].b",
+				createPayload("{\"a\":[{\"c\":1},{\"c\":2}]}"));
+	}
+
 	private void assertFieldType(JsonFieldType expectedType, String jsonValue)
 			throws IOException {
 		assertThat(this.fieldTypeResolver.resolveFieldType("field",