Using padding: ${'.3rem'} 0; not work

Bug report;

#### What is the current behavior?

``` css
.inner {
padding: ${'.3rem'} 0; // not work
//padding: ${`${px2rem(30)} 0`}; //good
}
```

#### What is the expected behavior?

#### Environment (include versions)

* OS: mac os mojave
* Browser: chrome
* styled-jsx (version): 3.2

Reproduce the reported styled-jsx-plugin-sass case using the shown `.inner` rule and compare the literal `${'.3rem'}` value with the `px2rem` interpolation. Trace how Sass and styled-jsx process each form, then verify that the padding declaration compiles and behaves as expected in Chrome.

Written by the indexing model from the issue text.