DefaultResolveFn: Use case insensitive comparison

[abihf]

To be able to use struct type for resolver without using field tags

schema:

type User {
  id: ID!
  name: String
}

Go:

type User struct {
  ID string
  Name string
}

[coveralls]

Coverage decreased (-0.01%) to 81.037% when pulling 0b39fb1 on abihf:patch-1 into 53935ab on graphql-go:master.

[chris-ramon]

LGTM 👍 — thanks a lot @abihf