Set-AzureRmApplicationGatewayBackendHttpSettings resets Request Timeout to 30 seconds when RequestTimeout parameter is not set

### Cmdlet(s)

Set-AzureRmApplicationGatewayBackendHttpSettings

### PowerShell Version
Instructions: to get PowerShell version, type `$PSVersionTable` and look for the value associated with `PSVersion`

5.1.14393.1770

### Module Version

### OS Version
Instructions: to get OS version, type `$PSversionTable` and look for value associated with `BuildVersion`

10.0.14393.1770

### Description

Set-AzureRmApplicationGatewayBackendHttpSettings overrides Request Timeout to 30 seconds when RequestTimeout parameter is not specified.

I have an existing backend http settings with 120 seconds timeout. I then call Set-AzureRmApplicationGatewayBackendHttpSettings to change the port of that setting, without passing the RequestTimeout parameter.

According to documentation the RequestTimeout parameter is optional and has no default value.

Howerver in my case the call to Set-AzureRmApplicationGatewayBackendHttpSettings resets the request timeout back to 30 seconds.

My workaround for now is to specify the optional RequestTimeout parameter.

### Debug Output
Instructions: to get Debug Output, set `$DebugPreference="Continue"` and then execute the cmdlet or script causing the issue

### Script/Steps for Reproduction