Merge pull request #468 from vladimir-poleh/master

Fix 'Write-Prompt' to be able use Black as foreground color.

src/GitPrompt.ps1

@@ -132,7 +132,7 @@ if (Get-Module NuGet) {
 
 function Write-Prompt($Object, $ForegroundColor = $null, $BackgroundColor = $null) {
     $s = $global:GitPromptSettings
-    if ($s -and !$ForegroundColor) {
+    if ($s -and ($null -eq $ForegroundColor)) {
         $ForegroundColor = $s.DefaultForegroundColor
     }