Fix for controlnet(integrated)'s api

[papuSpartan]

Description

Currently, sending an api request to controlnet integrated may result in issues. This is due to the base64 encoded images being accidentally left in base64 format. A check fails here because unit.image (and later unit.mask_image also) are always of type dict. The original comparison looks to be checking if contents are type str to infer if they are encoded. In practice, this fails without checking the image key for each which should actually contain the b64 image string.

May resolve: #439, #259, papuSpartan/stable-diffusion-webui-distributed#25

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

[papuSpartan]

Had a misunderstanding, unit formatting for the api should be like this per #162

{
  "controlnet": {
    "args": [
      {
        "image": "<base64string>",
        "image_mask": "<base64string>",
        ...
      },
      {
        "image": "<base64string>",
        "image_mask": "<base64string>",
        ...
      }
    ]
  }
}

unit.image: str is analogous to unit.input_image: str in mikubill's controlnet but I'm unsure as to why this was changed.