Added a regression test for #2399.
Initialize reader_t::items_index
Check pseudo types before inserting them into a table.
Remove unused literals from results of merge
Added another test case to the 2399 regression ReQL test.
Reworked datum_t::drop_literals()
Reverted datum validation
drop_literals() doesn't copy the datum anymore if it doesn't encounter a literal.
Added r_sanity_check for nested literal keywords.
Improved #2399 regression test.