Support mapping to json

[Gunni]

Can you support having a mapping converted to json?

Here is an example snippet from your readme:

    - name: create-json
      id: create-json
      uses: jsdaniell/create-json@v1.2.3
      with:
        name: "new-json-file.json"
        json: '{"name":"jsdaniell", "password":"mypass"}'

Could it be done like this?

    - name: create-json
      id: create-json
      uses: jsdaniell/create-json@v1.2.3
      with:
        name: "new-json-file.json"
        json:
          name: "jsdaniell"
          password: "mypass"

Today I get A mapping was not expected