[PR] Update Buffer.fill to accept a Buffer value
[PR] Update Buffer.fill to accept a Buffer value

Summary:
[`Buffer.fill`](https://nodejs.org/api/buffer.html#buffer_buf_fill_value_offset_end_encoding) accepts a `Buffer` for the value argument.

Currently, `Buffer.alloc(1).fill(Buffer.alloc(1));` results in the error

```
1: Buffer.alloc(1).fill(Buffer.alloc(1));
   ^ call of method `fill`. Function cannot be called on any member of intersection type
1: Buffer.alloc(1).fill(Buffer.alloc(1));
   ^ intersection
```

https://flow.org/try/#0EIVwZmCmBOB0CGAbRB7AxgCgIwEpZgEtkNQIYFl1sccBuIA
Closes https://github.com/facebook/flow/pull/3960

Differential Revision: D5108512

Pulled By: mroch

fbshipit-source-id: f3458b4c08356fe65f208f278e8fabd245f20859

TiddoLangerak/flow,popham/flow,AgentME/flow,MichaelDeBoey/flow,JonathanUsername/flow,jamesgpearce/flow,JonathanUsername/flow,gabelevi/flow,JonathanUsername/flow,ylu1317/flow,gabelevi/flow,JonathanUsername/flow,claudiopro/flow,popham/flow,AgentME/flow,samwgoldman/flow,nmote/flow,jamesgpearce/flow,fletcherw/flow,facebook/flow,ylu1317/flow,nmote/flow,MichaelDeBoey/flow,claudiopro/flow,nmote/flow,gabelevi/flow,nmote/flow,AgentME/flow,jamesgpearce/flow,JonathanUsername/flow,gabelevi/flow,nmote/flow,TiddoLangerak/flow,AgentME/flow,samwgoldman/flow,gabelevi/flow,mroch/flow,fletcherw/flow,fletcherw/flow,MichaelDeBoey/flow,ylu1317/flow,nmote/flow,jamesgpearce/flow,JonathanUsername/flow,jamesgpearce/flow,claudiopro/flow,facebook/flow,ylu1317/flow,samwgoldman/flow,mroch/flow,nmote/flow,AgentME/flow,facebook/flow,facebook/flow,mroch/flow,fletcherw/flow,TiddoLangerak/flow,popham/flow,samwgoldman/flow,popham/flow,mroch/flow,facebook/flow,TiddoLangerak/flow,facebook/flow,ylu1317/flow,popham/flow,gabelevi/flow,samwgoldman/flow,MichaelDeBoey/flow,samwgoldman/flow,facebook/flow,jamesgpearce/flow,gabelevi/flow,fletcherw/flow,mroch/flow,samwgoldman/flow,mroch/flow,claudiopro/flow,popham/flow,jamesgpearce/flow,fletcherw/flow,claudiopro/flow,TiddoLangerak/flow,popham/flow,MichaelDeBoey/flow,JonathanUsername/flow,mroch/flow