[SCSS] Empty spaces are removed from function argument strings in string interpolation

**Prettier 2.0.5**

**Input:**
```scss
$test: '#{string.index('this is a test', 'a test')}';
```

**Output:**
```scss
$test: '#{string.index('thisisatest', 'atest')}';
```

**Expected Behavior:**
Function arguments should not be altered because this breaks the code.