Fixed typo and enabled model change from txt2img and img2img

[iamianM]

POST was calling get function so I changed to set function.

Also added ability for txt2img and img2img to change loaded model by calling same set function. I feel this should be included as you can load models in memory so it doesn't have to take a long time.

[ClashSAN]

@iamianM please try again, the commit log is messy

[dfaker]

This is a mess, try a new branch with more focused changes, closing this one as it'll never be merged in this state.