Custom encoders don't work properly for struct pointers

[Timur Khametov]

Hello there. I've bumped into the bug related to encoding query if the struct contains reference to time.Time structure. I have described the problem here: https://github.com/gorilla/schema/issues/173 and there's also similar issue: https://github.com/gorilla/schema/issues/164 and created corresponding PR to fix it. Could somebody look at it please?
https://github.com/gorilla/schema/pull/174