Relax field type validation when marked as optional and value is null

Previously, if a field was marked as optional and it appeared in a
payload with a null value a failure would occur if the field's type
was configured to be a JsonFieldType other than NULL.

This commit relaxes this restriction so that a null value is tolerated
when a field has been marked as optional.

Closes gh-365

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2016 the original author or authors.
+ * Copyright 2014-2017 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.
@@ -108,7 +108,9 @@ class JsonContentHandler implements ContentHandler {
 			JsonFieldType actualFieldType = this.fieldTypeResolver
 					.resolveFieldType(fieldDescriptor.getPath(), readContent());
 			if (descriptorFieldType == JsonFieldType.VARIES
-					|| descriptorFieldType == actualFieldType) {
+					|| descriptorFieldType == actualFieldType
+					|| (fieldDescriptor.isOptional()
+							&& actualFieldType == JsonFieldType.NULL)) {
 				return descriptorFieldType;
 			}
 			throw new FieldTypesDoNotMatchException(fieldDescriptor, actualFieldType);