Fix an issue with unmatched param.

Hi @alexmingoia,

After playing around with `Router#param()` and `Route#param()` in your `koa-router` package, I encountered an issue with them.

In the condition for checking **matched* params, you use:

```
return routeParam.name = param;
```

instead of:

```
return routeParam.name == param;
```

so all params, whatever their name is, will be matched.

I created an additional test and fix this issue, please review it.