New-BasicAuthHeader.ps1

<#
    .SYNOPSIS
        Creates an HTTP header using BASIC authentication for use with Invoke-WebRequest or Invoke-RestMethod.

    .DESCRIPTION
        PowerShell 5 and below does not support BASIC authentication with Invoke-WebRequest and Invoke-RestMethod.
        This function will create a header in the format necessary to send BASIC authorization strings for HTTP
        requests. The returned header defaults to a hashtable, but with the -AsJson switch, it will return 
        properly formatted JSON.

        Optionally, an Accept and Content-Type can be provided.

    .PARAMETER Credential
        A PSCredential object consisting of the username and password used for BASIC authentication.

    .PARAMETER Accept
        A string representing the HTTP Accept necessary for interacting with the remote server.

    .PARAMETER ContentType
        A string representing the HTTP Content-Type being sent to the remote server.

    .PARAMETER AsJson
        When provided, returns a JSON object, instead of a hashtable.

    .EXAMPLE
        PS C:\> $Credential = Get-Credential
        PS C:\> New-BasicAuthHeader -Credential $Credential

        Name                           Value
        ----                           -----
        Authorization                  Basic c3lzdGHjasoKjlkIZXN0cmF0aW9uOlY0c0JRTzBkS3E3ZnJKLioUghNjGo==

        The example above a hashtable containing a Key called Authorization with a Value of the 
        Authorization string created from the credential passed in.

    .EXAMPLE
        PS C:\> $Credential = Get-Credential
        PS C:\> New-BasicAuthHeader -Credential $cred -Accept 'application/json' -ContentType 'application/json'

        Name                           Value
        ----                           -----
        Authorization                  Basic c3lzdGHjasoKjlkIZXN0cmF0aW9uOlY0c0JRTzBkS3E3ZnJKLioUghNjGo==
        Content-Type                   application/json
        Accept                         application/json

    .EXAMPLE
        PS C:\> $Credential = Get-Credential
        PS C:\> New-BasicAuthHeader -Credential $cred -Accept 'application/json' -ContentType 'application/json' -AsJson

        {
            "Authorization":  "Basic c3lzdGHjasoKjlkIZXN0cmF0aW9uOlY0c0JRTzBkS3E3ZnJKLioUghNjGo==",
            "Content-Type":  "application/json",
            "Accept":  "application/json"
        }

#>
function New-BasicAuthHeader {
    param(
        [Parameter( Mandatory )]
        [pscredential] $Credential,
        [string] $Accept,
        [string] $ContentType,
        [switch] $AsJson
    )

    process {
        $Header = @{}

        # Process and convert the credential into a BasicAuth string
        $Bstr = [System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($Credential.Password)
        $BasicPlain = '{0}:{1}' -f $Credential.UserName, ([System.Runtime.InteropServices.Marshal]::PtrToStringAuto($Bstr))
        $BasicB64   = [System.Convert]::ToBase64String([System.Text.Encoding]::UTF8.GetBytes($BasicPlain))
        $Header.Add('Authorization',('Basic {0}' -f $BasicB64))

        if ($Accept) {
            $Header.Add('Accept', $Accept)
        }

        if ($ContentType) {
            $Header.Add('Content-Type', $ContentType)
        }

        if ($AsJson) {
            Write-Output ($Header | ConvertTo-Json)
        }
        else {
            Write-Output $Header
        }
    }
}