code: public <T> AsyncMutateInBuilder arrayAddUnique(String path, T value) {
          this.mutationSpecs.add(new MutationSpec(Mutation.ARRAY_ADD_UNIQUE, path, value));
          return this;
      }
docs: Insert a value in an existing array only if the value
  isn't already contained in the array (by way of string comparison).
  
  @param path the path to mutate in the JSON.
  @param value the value to insert.