AppSettings string value is changed by Set-AzWebApp

## Description

I am using __Set-AzWebApp__ to set app settings on a web app I create dynamically (using __New-AzWebApp__). One of the settings is the web app creation time (as I did not find any other way how to get this information using Azure PowerShell). I am using the neutral round-trip format of the datetime to be able to reliably parse the string later. However, even though the string looks as expected locally, on the web app it is in a different format:

```
On the DevOps agent
2020-01-15T16:43:58.2385875Z

In the web app AppSetting
01/15/2020 16:45:06
```

## Steps to reproduce

```powershell

$AppSettings = @{
"WEBSITE_SITE_TYPE" = "live"
"CREATED_UTC" = [DateTime]::UtcNow.ToString("o", [CultureInfo]::InvariantCulture)
}

Write-Host (ConvertTo-Json $AppSettings)

Set-AzWebApp -ResourceGroupName $TargetResourceGroup -Name $TargetAppName -AppSettings $AppSettings
```

The __Write-Host__ command outputs

```
{
"WEBSITE_SITE_TYPE": "live",
"CREATED_UTC": "2020-01-15T16:43:58.0823349Z"
}
```

as expected. However, after the __Set-AzWebApp__ call, the web app shows


## Module versions

```powershell
=================
Task : Azure PowerShell
Description : Run a PowerShell script within an Azure environment
Version : 4.159.7
Author : Microsoft Corporation
Help : [Learn more about this task](https://go.microsoft.com/fwlink/?LinkID=613749)
=================

VERBOSE: Loading module from path
'C:\Modules\az_3.1.0\Az.Websites\1.5.0\Microsoft.Azure.PowerShell.Cmdlets.Websites.dll'
```

Start by reproducing the issue with Set-AzWebApp, New-AzWebApp, and the provided AppSettings example, comparing the local value with the value stored by the web app. Trace the Azure Web Apps API request used by Set-AzWebApp; done means the CREATED_UTC setting retains its invariant round-trip string rather than being reformatted.

Written by the indexing model from the issue text.