Merge pull request #101 in G/fastr from...

Merge pull request #101 in G/fastr from ~STEPAN.SINDELAR_ORACLE.COM/fastr:feature/small-fixes to master

* commit 'cfa8fcb3':
  Tests for parse with single identifier: parse(text='a')
  AsVector can handle RLanguage when mode='any'
  Fix eval to handle NULL as 'enclos'
  Fix RLanguage handling in `$<-` when the index does not exist
  UpdateAttr shows compatible error when lhs is NULL