* Make FlutterStandardCodec handle writing NSData
iOS developers can accidentally pass an `NSData` object directly to
`FlutterStandardCodec` and forget to wrap it in
`FlutterStandardTypedData`. This failure won't be caught until
runtime.
Let's make `FlutterStandardCodec` more tolerant by making it assume
that `NSData` should be treated as a binary blob and by wrapping it
automatically.
Fixes https://github.com/flutter/flutter/issues/17449
Testing Done:
* Made a sample Flutter iOS application that created a
`FlutterMethodChannel` and invoked a method using an `NSData*`
argument. Verified that the Flutter method handler received the
message and that the `MethodCall.arguments` was a `UInt8Array` that
preserved the original byte order.
* Verified that without this change, the same sample application
crashed when run in debug mode.
* Add a unit test for encoding raw NSData
* Formatting adjustments
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