Incorrect type for (js.type = "Date")

Thrift definition:
```
1: optional i64 (js.type = "Date") timestampMs;
```
Generated type:
```
timestampMs?: ?string,
```
while it should be:
```
timestampMs?: ?string | ?number,
```
According to https://github.com/thriftrw/thriftrw/blob/master/annotations.md#timestamps
> will accept an ISO-8601 timestamp or milliseconds since the epoch (as returned by `Date.now()`)
