def []=(key, value)
      new_entry = entry_to_modify(key, value)
      new_entry[:key] = key
      new_entry[:value] = value
    end
def []=(key, value)
      new_entry = entry_to_modify(key, value)
      new_entry[:key] = key
      new_entry[:value] = value
    end
Replace the first value for *key* with *value*.
 Append a new element if *key* did not exist.
 If *key* was previously removed, then put it back to its old position.
 @param key [String]
 @param value [String, AugeasTree, AugeasTreeValue]