Fixes a runtime error that occurs when deep-merging fragments

[namenu]

Fixes a bug where inconsistent results occur based on the fragment's declared order.

I fixed a similar issue in #453, but in that case the fragment was nested, whereas this case is not.

This fails:

query MyQuery {
  node(id: "1000") {
    ... on Post {
      id
      ...PostFragment
    }
  }
}

fragment PostFragment on Post {
  author {
    alwaysFail
  }
  ...PostFragment2   # <--
}

fragment PostFragment2 on Post {
  author {
    name
  }
}

While this is not:

query MyQuery {
  node(id: "1000") {
    ... on Post {
      id
      ...PostFragment
    }
  }
}

fragment PostFragment on Post {
  ...PostFragment2   # <--
  author {
    alwaysFail
  }
}

fragment PostFragment2 on Post {
  author {
    name
  }
}

My guess is that deep-merge doesn't handle the situation where nil comes in instead of map.

[namenu]

I observed that left/rigt can be ::null or nil.
It depends on whether the object that catches bubbled-up error is non-null or not.

Test case in L169 covers this.

[hlship]

I'll review this once you re-open it.

[namenu]

@hlship Thanks. We have encountered a regression bug after applying this changes. I am not confident much so that I can handle this problem since it requires profound understanding of the internal design.
I'm still finding the way how to reproduce the bug. If you have any idea about this please let me know.

[namenu]

Reopened #463