Make Get-Item env: case-aware on Unix

Fix #1196

src/System.Management.Automation/namespaces/EnvironmentProvider.cs

@@ -190,8 +190,15 @@ namespace Microsoft.PowerShell.Commands
         /// 
         internal override IDictionary GetSessionStateTable ()
         {
+            // Environment variables are case-sensitive on Unix and
+            // case-insensitive on Windows
+#if UNIX
+            Dictionary<string, DictionaryEntry> providerTable =
+                new Dictionary<string, DictionaryEntry>(StringComparer.Ordinal);
+#else
             Dictionary<string, DictionaryEntry> providerTable =
                 new Dictionary<string, DictionaryEntry>(StringComparer.OrdinalIgnoreCase);
+#endif
 
             // The environment variables returns a dictionary of keys and values that are 
             // both strings. We want to return a dictionary with the key as a string and

test/powershell/Modules/Microsoft.PowerShell.Management/Get-ChildItem.Tests.ps1

@@ -1,4 +1,7 @@
 Describe "Get-ChildItem" -Tags "CI" {
+
+    Context 'FileSystem provider' {
+
         It "Should list the contents of the current folder" {
             (Get-ChildItem .).Name.Length | Should BeGreaterThan 0
         }
@@ -34,3 +37,23 @@ Describe "Get-ChildItem" -Tags "CI" {
             $files[4].Name     | Should Be "D"
         }
     }
+
+    Context 'Env: Provider' {
+
+        It 'can handle mixed case in Env variables' {
+            try
+            {
+                $env:__FOOBAR = 'foo'
+                $env:__foobar = 'bar'
+
+                $foobar = Get-Childitem env: | ? {$_.Name -eq '__foobar'}
+                $count = if ($IsWindows) { 1 } else { 2 }
+                ($foobar | measure).Count | Should Be $count
+            }
+            catch
+            {
+                Get-ChildItem env: | ? {$_.Name -eq '__foobar'} | Remove-Item -ErrorAction SilentlyContinue
+            }
+        }
+    }
+}