Before this change, the following invocation would not work
```
$ http http://neverhttps.com <&-
```
The "<&-" at the end closes the stdin fd. Specifically, it would fail with
```
...
File "/home/mgsloan/.local/lib/python3.6/site-packages/httpie/context.py", line 26, in Environment
stdin_isatty = stdin.isatty()
AttributeError: 'NoneType' object has no attribute 'isatty'
```
This can occur when httpie is being programmatically invoked, and may
as well be supported.
