(GH-699) Fix: PATH may break on Install-ChocolateyPath

The fixes for GH-303 (e050c22) had an interesting and terrible side
effect. When you don't expand the value and then you update using
`[Environment]::SetEnvironmentVariable($Name, $Value, $Scope)`, it has
the side effect of overwriting the original registry value type and
saves the values as `REG_SZ` instead of keeping it as `REG_EXPAND_SZ`.
Then all of the tools in WinDir no longer work all of a sudden and
it's somewhat perplexing what happened as everything looks appropriate
when you inspect the values.

You can verify if you are affected by opening a new command line and
typing `where.exe choco` and the system cannot find where.exe, which is
at `C:\Windows\System32\where.exe`.

You are only affected if you installed (explicitly installed, not upgraded)
one of the following beta releases directly:

 * https://chocolatey.org/packages/chocolatey/0.9.10-beta-20160411
 * https://chocolatey.org/packages/chocolatey/0.9.10-beta-20160422
 * https://chocolatey.org/packages/chocolatey/0.9.10-beta-20160503

This is fixed by attempting to ask the registry what the value type is
for the particular key and falling back to `REG_SZ` when it is not
found. Environment variables are always either String or ExpandString. As
a further step, if it is the `PATH` variable, it will always be saved as
ExpandString (`REG_EXPAND_SZ`).

Then set the value using `[Microsoft.Win32.Registry]::SetValue` instead
because it allows setting the value type.

src/chocolatey.resources/helpers/functions/Set-EnvironmentVariable.ps1

@@ -14,5 +14,32 @@
 
 function Set-EnvironmentVariable([string] $Name, [string] $Value, [System.EnvironmentVariableTarget] $Scope) {
 	Write-Debug "Calling Set-EnvironmentVariable with `$Name = '$Name', `$Value = '$Value', `$Scope = '$Scope'"
-    [Environment]::SetEnvironmentVariable($Name, $Value, $Scope)
+
+  if ($Scope -eq [System.EnvironmentVariableTarget]::Process) {
+    return [Environment]::SetEnvironmentVariable($Name, $Value, $Scope)
+  }
+
+  [string]$keyHive = 'HKEY_LOCAL_MACHINE'
+  [string]$registryKey = "SYSTEM\CurrentControlSet\Control\Session Manager\Environment\"
+  [Microsoft.Win32.RegistryKey] $win32RegistryKey = [Microsoft.Win32.Registry]::LocalMachine.OpenSubKey($registryKey)
+  if ($Scope -eq [System.EnvironmentVariableTarget]::User) {
+    $keyHive = 'HKEY_CURRENT_USER'
+    $registryKey = "Environment"
+    [Microsoft.Win32.RegistryKey] $win32RegistryKey = [Microsoft.Win32.Registry]::CurrentUser.OpenSubKey($registryKey)
+  }
+
+  [Microsoft.Win32.RegistryValueKind]$registryType = [Microsoft.Win32.RegistryValueKind]::String
+  try { 
+    $registryType = $reg.GetValueKind($Name)
+  } catch { 
+    # the value doesn't yet exist
+    # move along, nothing to see here
+  }
+  Write-Debug "Registry type for $Name is/will be $registryType"
+
+  if ($Name -eq 'PATH') {
+    $registryType = [Microsoft.Win32.RegistryValueKind]::ExpandString
+  }
+
+  [Microsoft.Win32.Registry]::SetValue($keyHive + "\" + $registryKey, $Name, $Value, $registryType)
 }