hpack/src/main/java/com/twitter/hpack/Encoder.java
HeaderField getHeaderField(int index) {
    HeaderEntry entry = head;
    while(index-- >= 0) {
      entry = entry.before;
    }
    return entry;
  }
HeaderField getHeaderField(int index) {
    HeaderEntry entry = head;
    while(index-- >= 0) {
      entry = entry.before;
    }
    return entry;
  }
Return the header field at the given index.
Exposed for testing.