Don't overwrite passed props to mapDispatchToProps with the default

I found an unexpected behavior when defining second argument received by `mapDispatchToProps` to have a default value.
The default value is always passed to it, even though other props are defined on the component.

I created a test to demonstrate it: https://github.com/oskarhane/preact-redux/commit/b0a2b89b54a0061e760575070db15350f115e68c

If the default is removed:
`let mapDispatchToProps = (d, p = {}) => {` to
`let mapDispatchToProps = (d, p) => {`
it works as expected.