Don't fail to deserialize data_base64 even if datacontenttype lies

This fixes #160

From the spec: "When a CloudEvent is deserialized from JSON, the
presence of the data_base64 member clearly indicates that the value is
a Base64 encoded binary data, which the deserializer MUST decode into
a binary runtime data type. The deserializer MAY further interpret
this binary data according to the datacontenttype."
https://github.com/cloudevents/spec/blob/master/cloudevents/formats/json-format.md#312-payload-deserialization

Signed-off-by: Jim Crossley <jim@crossleys.org>

src/event/builder.rs

@@ -200,6 +200,10 @@ mod tests {
             fixtures::v10::full_json_base64_data_json(),
             fixtures::v10::full_json_data()
         ),
+        case::full_v10_with_non_json_base64_data(
+            fixtures::v10::full_non_json_base64_data(),
+            fixtures::v10::full_non_json_data()
+        ),
         case::full_v10_with_xml_string_data(
             fixtures::v10::full_xml_string_data_json(),
             fixtures::v10::full_xml_string_data()

src/event/v10/format.rs

@@ -42,7 +42,13 @@ impl crate::event::format::EventFormatDeserializer for EventFormatDeserializer {
         Ok(match (data, data_base64, is_json) {
             (Some(d), None, true) => Some(Data::Json(parse_data_json!(d, E)?)),
             (Some(d), None, false) => Some(Data::String(parse_data_string!(d, E)?)),
-            (None, Some(d), true) => Some(Data::Json(parse_json_data_base64!(d, E)?)),
+            (None, Some(d), true) => {
+                let dc = d.to_owned();
+                match parse_json_data_base64!(dc, E) {
+                    Ok(x) => Some(Data::Json(x)),
+                    Err(_) => Some(Data::Binary(parse_data_base64!(d, E)?)),
+                }
+            }
             (None, Some(d), false) => Some(Data::Binary(parse_data_base64!(d, E)?)),
             (Some(_), Some(_), _) => {
                 return Err(E::custom("Cannot have both data and data_base64 field"))

src/test/fixtures/v10.rs

@@ -170,6 +170,28 @@ pub fn full_json_base64_data_json() -> Value {
     })
 }
 
+pub fn full_non_json_base64_data() -> Value {
+    match full_json_base64_data_json() {
+        Value::Object(mut m) => {
+            m.insert(
+                "data_base64".to_string(),
+                Value::String(base64::encode(b"hello world")),
+            );
+            Value::Object(m)
+        }
+        _ => Value::Null,
+    }
+}
+
+pub fn full_non_json_data() -> Event {
+    let mut event = full_json_data();
+    let value = full_non_json_base64_data();
+    if let Value::Object(m) = value {
+        event.set_data_unchecked(base64::decode(m["data_base64"].as_str().unwrap()).unwrap());
+    }
+    event
+}
+
 pub fn full_xml_string_data() -> Event {
     let (string_ext_name, string_ext_value) = string_extension();
     let (bool_ext_name, bool_ext_value) = bool_extension();