Material-ui: [Button] Variant outlined has same class duplicated

[drianoaz]

• [x] The issue is present in the latest release.
• [x] I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

The MuiButton-outline class has been twice applied in button component. When I updated to latest version in my project, the snapshots has broken.

Expected Behavior 🤔

Apply only one class MuiButton-outline in outlined button

Steps to Reproduce 🕹

Steps:

1. You can see this Behavior in documentation of button, when inspect element on dev tools

Context 🔦

Your Environment 🌎

| Tech | Version |
| ----------- | ------- |
| Material-UI | v4.5.0 |
| Browser | Chrome 76.0.3809.132 |

[mbrookes]

Dupe of #17667 ?

[oliviertassinari]

Fix released in v4.5.1, we should be good.