[merge options] Inconsistent behavior between 0.18.1 and 0.19.0

[liyuanqiu]

Describe the bug
A clear and concise description of what the bug is. If your problem is not a bug, please file under Support or Usage Question

v0.19.0 changes the source code of merging default options and request options.
But the merging behavior wasn't kept same with v0.18.1.

For example, if default options is:

{
  baseUrl: 'some_url',
  params: {
    foo: 'bar',
  },
}

In v0.18.1, the merged options will contain `params`, but in v0.19.0, the merged options won't contain `params`.

To Reproduce
Code snippet to reproduce, ideally that will work by pasting into something like https://npm.runkit.com/axios, a hosted solution, or a repository that illustrates the issue. If your problem is not reproducible, please file under Support or Usage Question

const instance = axios.create({
  params: {
    foo: 'bar',
  },
});
instance({
  url: 'some_url',
  method: 'get',
});

// in v0.18.1
// GET some_url?foo=bar
// https://codesandbox.io/s/charming-wilson-9wkge

// in v0.19.0
// GET some_url
// https://codesandbox.io/s/nifty-fire-yhhth

Expected behavior
A clear and concise description of what you expected to happen.

I think v0.19.0 shouldn't change the merging behavior.

Environment:

• Axios Version [0.19.0]
• OS: [OSX 10.14.6]
• Browser [Chrome]
• Browser Version [Version 76.0.3809.100 (Official Build) (64-bit)]
• Additional Library Versions [React 16.9]

Additional context/Screenshots
Add any other context about the problem here. If applicable, add screenshots to help explain.

[G-Rath]

I've just hit this, and found it very surprising. It goes against the use-case of create:

const axios = Axios.create({
  baseURL: url.resolve(process.env.API_URL, '/api'),
  params: { token: process.env.API_TOKEN }
});

While params is less likely to be used compared to say auth or header, at best it's inconsistent - I'm now scared that any property I provide when calling create will actually be used at all, and so have to spend time figuring out what'll happen for every property.

[reinoute]

Same here. My project broke when updating to 1.9, so I had to revert.

[willxy]

possibly related to: #2190

[elboletaire]

possibly related to: #2190

It is indeed the same issue described in #2190

[chinesedfan]

So let's track in #2190. Hope someone can merge and release that soon.