[SPARK-4314][Streaming] Exception when textFileStream attempts to read deleted _COPYING_ file

The ephemeral file(\_COPYING\_) is caught by FileInputDStream interface.
Exception appears when textFileStream attempts to read deleted \_COPYING\_ file if \_COPYING\_ is deleted when spark handle this file.
Therefore, add filter for intermediate partly written state 
It's pretty minor, but to me this is clearer in the more compact form:
```
def defaultFilter(path: Path): Boolean = 
  !path.getName.startsWith(".") && !path.getName.endsWith("_COPYING_")
```