Prototypes are lost when set as theme defaults

[remcohaszing]

• This is a v1.x issue (v0.x is no longer maintained).
• I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior

Default props in a theme keep their prototype.

Current Behavior

The prototype in theme props is removed due to the use of deepmerge.

Steps to Reproduce (for bugs)

The following works:

const popoverContainer = document.createElement('div');

<MuiThemeProvider
  theme={createMuiTheme({
    props: {
      MuiPopover: {
        container: () => popoverContainer,
      },
    },
  })}
>
  <App />
</MuiThemeProvider>

The following does not (but should):

const popoverContainer = document.createElement('div');

<MuiThemeProvider
  theme={createMuiTheme({
    props: {
      MuiPopover: {
        // The popover will receive a regular object
        container: popoverContainer,
      },
    },
  })}
>
  <App />
</MuiThemeProvider>

https://codesandbox.io/s/04j9lxymow (based on #12011 (comment))

1. Pass an HTML element as a MuiPopover prop to <MuiThemeProvider />
2. Render a <Popover />
3. Notice the app crashes.

Context

https://material-ui.com/customization/themes/#properties

Your Environment

Tech	Version
Material-UI	v1.3.0
React	v16.3.0
browser	Chrome 67
deepmerge	v2.1.1

[oliviertassinari]

The prototype in theme props is removed due to the use of deepmerge.

@remcohaszing Looking at the options of deepmerge, I think that we should be ignoring non "plain object" : https://github.com/KyleAMathews/deepmerge#ismergeableobject.
This implementation is intereseting: https://github.com/jonschlinkert/is-plain-object/blob/master/index.js.

[kivlor]

I've opened a WIP PR here - #12100