Fix issue 443 plugin proxy_cache got "expected an integer"

[danybmx]

This MR fix the problem from #433 when trying to add the proxy_cache plugin to a route, service or globally.

It just transforms the string elements of the array generated by chips with integers when the field elements type is integer.

Steps to test:

1. Go to Plugins
2. Add a new Global Plugin and use Others > Proxy Cache
3. Write some response codes 200 304...
4. Select strategy > memory
5. Save.

Hope it helps

[pantsel]

@danybmx , thanks a bunch for the contribution man!

[Jeiwan]

@pantsel Could you please release a new version with this change?