fix: Ensure params aren't copied between partial matches

[rschristian]

Fixes #123

Because each call of exec shared the same params obj, partial matches could get carried between attempts to find a matching child.

<Route path="/category/:id" component={Params} />
<Route path="/category/:categoryId/products/new" component={Params} />

When trying to load '/category/123/products/new', it'd test the first route, setting params up as { id: '123' }, and when that failed, the same params object was passed to the second test. It of course would match, but the params object would end up as { id: '123', categoryId: '123' }, keeping the incorrect id from the previous failed match.

[rschristian]

This Object.assign() is the only change, rest is just formatting as that line was getting rather long & hard to read.