Support working with CHI automatically if Cache::FileCache is not installed

I've had recurrent problems with Cache::FileCache failing its test during install, requiring manual intervention to get WWW::Mech installed.

This patch makes it so that in the event you somehow opt to install WWW::Mechanize::Cached without Cache::FileCached installed, then tests will still pass assuming CHI is installed ( which proves to be more reliable these days )

An additonal test is added to explicitly test CHI when it is installed.

**Note:** This does not presently change any dependencies.

I considered one could make **both** `CHI` and `Cache::FileCache` `recommended` deps and have WWW::Mech install without dependencies ( assuming I found the right glue to make it work internally ), but the way I've adjusted it so far means you need at least one of the two for anything to work.

So you could probably just alternate between 
```
runtime.requires = Cache::FileCache
runtime.recommends = CHI
```
and
```
runtime.requires = CHI
runtime.recommends = Cache::FileCache
```

Based on whim and everything should just work. Users who desire explict behaviour should be hardcoding it anyway.