Feature Request : support streams for non-json content payloads

I have the following openapi.yml:
```yml
openapi: 3.0.1
info:
title: MtRainier
description: Gets whether the mountain is out!
version: 1.0.0
servers:
- url: https://themtn.azurewebsites.net/
paths:
/:
get:
summary: Gets whether the mountain is out!
operationId: MtRainer_invoke
responses:
200:
description: OK
content:
text/html:
schema:
type: string
/api:
get:
summary: Gets whether the mountain is out!
operationId: MtRainer_get
responses:
200:
description: OK
content:
application/json:
schema:
$ref: "#/components/schemas/MtRainer"
components:
schemas:
MtRainer:
type: object
properties:
result:
type: boolean
image:
type: string
```

Note, the `/` route (ignore the `/api` route).
It returns a:
```
text/html:
schema:
type: string
```

When I generate and run the command, I get an NullRef.:
```
❯❯❯ autorest --input-file=./openapi.yml --powershell

AutoRest code generation utility [version: 3.0.5166; node: v10.15.0]
(C) 2018 Microsoft Corporation.
https://aka.ms/autorest
Loading AutoRest core '/Users/tyler/.autorest/@microsoft.azure_autorest-core@3.0.5402/node_modules/@microsoft.azure/autorest-core/dist' (3.0.5402)
Loading AutoRest extension '@microsoft.azure/autorest.powershell' (beta->2.0.347)
Loading AutoRest extension '@microsoft.azure/autorest.remodeler' (beta->2.0.213)
Loading AutoRest extension '@microsoft.azure/autorest.csharp-v2' (beta->2.0.227)
--------------------------------------------------------------------

❯❯❯ cd ./generated/

❯❯❯ ./build-module.ps1 -Run

Creating isolated process...
Cleaning build folders...
Compiling module...
-------------Done-------------
Creating isolated process...

PS /Users/tyler/Code/PowerShell/AutoRest/IsTheMtnOut/generated [MtRainier]> Invoke-InvokeMtRainer

Invoke-InvokeMtRainer : Object reference not set to an instance of an object.
At line:1 char:1
+ Invoke-InvokeMtRainer
+ ~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [Invoke-InvokeMtRainer], NullReferenceException
+ FullyQualifiedErrorId : Sample.API.Cmdlets.InvokeInvokeMtRainer

PS /Users/tyler/Code/PowerShell/AutoRest/IsTheMtnOut/generated [MtRainier]> $Error[0].Exception.StackTrace

at Sample.API.Cmdlets.InvokeInvokeMtRainer.onOK(HttpResponseMessage responseMessage, Task`1 response) in /Users/tyler/Code/PowerShell/AutoRest/IsTheMtnOut/generated/generated/cmdlets/InvokeInvokeMtRainer.cs:line 269
at Sample.API.MtRainier.MtRainerInvoke_Call(HttpRequestMessage request, Func`3 onOK, IEventListener eventListener, ISendAsync sender) in /Users/tyler/Code/PowerShell/AutoRest/IsTheMtnOut/generated/generated/api/MtRainier.cs:line 146
at Sample.API.MtRainier.MtRainerInvoke_Call(HttpRequestMessage request, Func`3 onOK, IEventListener eventListener, ISendAsync sender) in /Users/tyler/Code/PowerShell/AutoRest/IsTheMtnOut/generated/generated/api/MtRainier.cs:line 161
at Sample.API.MtRainier.MtRainerInvoke(Func`3 onOK, IEventListener eventListener, ISendAsync sender) in /Users/tyler/Code/PowerShell/AutoRest/IsTheMtnOut/generated/generated/api/MtRainier.cs:line 119
at Sample.API.Cmdlets.InvokeInvokeMtRainer.ProcessRecordAsync() in /Users/tyler/Code/PowerShell/AutoRest/IsTheMtnOut/generated/generated/cmdlets/InvokeInvokeMtRainer.cs:line 226
at Sample.API.Cmdlets.InvokeInvokeMtRainer.ProcessRecordAsync() in /Users/tyler/Code/PowerShell/AutoRest/IsTheMtnOut/generated/generated/cmdlets/InvokeInvokeMtRainer.cs:line 232
PS /Users/tyler/Code/PowerShell/AutoRest/IsTheMtnOut/generated [MtRainier]>
```

Side issue... I expected the cmdlet name to be `Invoke-MtRainier` not `Invoke-InvokeMtRainer`...

FWIW, the `Get-MtRainier` cmdlet works with the output type of object.