Fix isJSON for AWS content type

[fxalgrain]

I create this PR to deals with the following issue.

When working with some AWS API, you have to use the "application/x-amz-json-1.1" content type.
The issue is this content type is not recognized as JSON and so, you can not bind the result to variable.
Instead you get the following error : "Only JSON and XML response/request body is supported to query the result"

You can reproduce the issue with this snippet:

# @name login
POST https://cognito-idp.eu-west-1.amazonaws.com
Content-Type: application/x-amz-json-1.1
X-Amz-Target: AWSCognitoIdentityProviderService.InitiateAuth


{
   "AuthParameters" : {
      "USERNAME" : "<user>",
      "PASSWORD" : "<pwd>"
   },
   "AuthFlow" : "USER_PASSWORD_AUTH",
   "ClientId" : "123456789"
}
@id_token = {{login.response.body.AuthenticationResult.IdToken}}

[Huachao]

@fxalgrain Merged, thanks

[fxalgrain]

@fxalgrain Merged, thanks

You're welcome, but it's me to thanks you about your amazing work on this tool!

Have a good day