Fix minification issues

[Igorbek]

This PR fixes #36

Example:

styled.div`
  border: ${'solid'} 2px;
`

Minified before:

styled.div`border:${'solid'}2px;` // would result in 'border:solid2px'

Minified after:

styled.div`border:${'solid'} 2px;` // would result in 'border:solid 2px'

Particularly these two commits show the diff:

• Fix 1: 0b7dfc4
• Fix 2: a8bee3f