/**
   * Returns the first element of an observable sequence that satisfies the condition in the predicate if present else the first item in the sequence.
   * @example
   * var res = res = source.first();
   * var res = res = source.first(function (x) { return x > 3; });
   * @param {Function} [predicate] A predicate function to evaluate for elements in the source sequence.
   * @param {Any} [thisArg] Object to use as `this` when executing the predicate.
   * @returns {Observable} Sequence containing the first element in the observable sequence that satisfies the condition in the predicate if provided, else the first item in the sequence.
   */
  observableProto.first = function (predicate, thisArg) {
    return predicate ?
      this.where(predicate, thisArg).first() :
      firstOrDefaultAsync(this, false);
  };