Composing function styles causes dynamic styles to not be used

As documented in [the readme](https://github.com/cssinjs/styled-jss#composable-styles), you are able to compose style-objects or style-functions by passing additional arguments to `styled`.

However, if you pass a style-function then any function rules in the base styles will not be respected

see example:
https://codesandbox.io/embed/jss-composable-styles-bug-rt1on

Start with the composable-styles section of the README and reproduce the behavior using the linked CodeSandbox. Trace how a style-function is composed with base styles, then add coverage for function rules in the base styles. Done means those dynamic rules are applied when an additional style-function is passed to styled.

Written by the indexing model from the issue text.