[ruby-core:97941] [Ruby master Bug#16798] ENV.replace on Windows with a variable name not respecting case will clear it

Issue #16798 has been reported by deivid (David Rodr=EDguez).

----------------------------------------
Bug #16798: ENV.replace on Windows with a variable name not respecting case=
 will clear it
https://bugs.ruby-lang.org/issues/16798

* Author: deivid (David Rodr=EDguez)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
On Windows, environment variable names are case insensitive:

```
PS C:\Users\deivi> ruby -e "puts ENV['Path']; ENV['PATH'] =3D 'foo'; puts E=
NV['Path']"                                                                =
                                                                           =
          C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Window=
s\System32\WindowsPowerShell\v1.0\;C:\Windows\System32\OpenSSH\;C:\Program =
Files\Git\cmd;C:\Program Files\Git\mingw64\bin;C:\Program Files\Git\usr\bin=
;C:\Ruby26-x64\bin;C:\Ruby25-x64\bin;C:\Windows\system32;C:\Windows;C:\Wind=
ows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\Sy=
stem32\OpenSSH\;C:\Program Files\Git\cmd;C:\Program Files\Git\mingw64\bin;C=
:\Program Files\Git\usr\bin;C:\Windows\system32;C:\Windows;C:\Windows\Syste=
m32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\System32\Op=
enSSH\;C:\Program Files\Git\cmd;C:\Program Files\Git\mingw64\bin;C:\Program=
 Files\Git\usr\bin;C:\Ruby27-x64\bin;C:\Ruby25-x64\bin
foo

PS C:\Users\deivi> ruby -e "puts ENV['Path']; ENV.update('PATH' =3D> 'foo')=
; puts ENV['Path']"                                                        =
                                                                           =
          C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Window=
s\System32\WindowsPowerShell\v1.0\;C:\Windows\System32\OpenSSH\;C:\Program =
Files\Git\cmd;C:\Program Files\Git\mingw64\bin;C:\Program Files\Git\usr\bin=
;C:\Ruby26-x64\bin;C:\Ruby25-x64\bin;C:\Windows\system32;C:\Windows;C:\Wind=
ows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\Sy=
stem32\OpenSSH\;C:\Program Files\Git\cmd;C:\Program Files\Git\mingw64\bin;C=
:\Program Files\Git\usr\bin;C:\Windows\system32;C:\Windows;C:\Windows\Syste=
m32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\System32\Op=
enSSH\;C:\Program Files\Git\cmd;C:\Program Files\Git\mingw64\bin;C:\Program=
 Files\Git\usr\bin;C:\Ruby27-x64\bin;C:\Ruby25-x64\bin
foo
```

However, with `ENV.replace`, if you don't use the "canonical casing" (as gi=
ven by `ENV.keys`, for example), the environment variable will be cleared.

```
PS C:\Users\deivi> ruby -e "puts ENV['Path']; ENV.replace('PATH' =3D> 'foo'=
); puts ENV['Path']"                                                       =
                                                                           =
               C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\W=
indows\System32\WindowsPowerShell\v1.0\;C:\Windows\System32\OpenSSH\;C:\Pro=
gram Files\Git\cmd;C:\Program Files\Git\mingw64\bin;C:\Program Files\Git\us=
r\bin;C:\Ruby26-x64\bin;C:\Ruby25-x64\bin;C:\Windows\system32;C:\Windows;C:=
\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windo=
ws\System32\OpenSSH\;C:\Program Files\Git\cmd;C:\Program Files\Git\mingw64\=
bin;C:\Program Files\Git\usr\bin;C:\Windows\system32;C:\Windows;C:\Windows\=
System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\System=
32\OpenSSH\;C:\Program Files\Git\cmd;C:\Program Files\Git\mingw64\bin;C:\Pr=
ogram Files\Git\usr\bin;C:\Ruby27-x64\bin;C:\Ruby25-x64\bin


```

This is unexpected to me.



-- =

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