Rxjs: endWith should widen the type if necessary

[cartant]

Feature Request

startWith can be passed values other than T and if that's done, the operator's return type is widened. See #4034.

endWith needs to be treated similarly and needs an additional dtslint test, etc.

[cartant]

@dkosasih If you want to handle this, that would be great.

[timdeschryver]

Feel free to ping me if you need to verify something @dkosasih

[cartant]

A PR for this has been merged.