Make default transformResponse parse JSON more properly

[chinesedfan]

Similar PRs before,

• Fixing parse by responseType  #2424, parse JSON in adapters
• Fixing sometimes response.data is string instead of object #2849, throw errors for invalid JSONs if content-type is application/json

Not sure this is the best solution, because it changes transformers with a new parameter and will break codes depending on silent parsing. So I will not fix tests until we have determined to do.

• Now the default transformResponse will not parse the string as JSON if responseType is text. Fixes responseType=text but json is parsed #2791 and several issues in Use the proper transformers for specific dataType and responseType values. #811 (comment).

• But once tried, invalid JSON string will cause axios rejects. Fixes How to handle error from JSON.parse #61.

[Alanscut]

@chinesedfan Should we just pass the config.responseType parameter? And also need to update README.md.

  transformRequest: [function (data, headers) {
    // Do whatever you want to transform the data

    return data;
  }],

[micate]

Should we just pass the config.responseType parameter?

Pass config has more applicable scenes, eg #842

[fnune]

This also fixes #907 btw