Extra constant fix

[devkabiir]

This will fix the bug when

@GET('/list')
@dio.Extra({'my_key': CustomValue()}
Future<void> list();

Produces the extra variable with null values

const _extra = <String, dynamic>{'key':null};

This will fix it and produce correct output

const _extra = <String, dynamic>{'key': CustomValue()};

Also added test cases for this

[devkabiir]

@trevorwang You should add dart: stable in Travis config and also run pub run test.

[trevorwang]

@devkabiir Will enable in another commit