$splatParameters = @{
              Uri             = "https://powershell.atlassian.net/wiki/rest/api/content/13009245?expand=space,version,body.storage,ancestors&limit=25"
              Method          = "GET"
              Headers         = @{
  "Accept-Charset"                = "utf-8"
  "Accept"                        = "application/json"
  "Content-Type"                  = "application/json; charset=utf-8"
  "Authorization"                 = "Basic abcd"
  }
              UseBasicParsing = $true
              ErrorAction     = 'SilentlyContinue'
          };
  Set-Content -Value (Invoke-WebRequest @splatParameters).Content -Path c:\temp\export.txt -Encoding UTF8 -force

PS C:\Program Files\PowerShell\6.0.0-rc> $PSVersiontable

Name                           Value
----                           -----
PSVersion                      6.0.0-rc
PSEdition                      Core
GitCommitId                    v6.0.0-rc
OS                             Microsoft Windows 10.0.15063
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0