[ADQL] Fix escaping of double quotes in delimited identifiers.

A delimited identifier is any sequence of characters between a pair of
double quotes. For instance: "123 I am a delimited identifier!".

It is of course possible to have double quotes inside this kind of identifier,
but they have to be doubled in order to not be mistaken with the end of the
identifier. For instance: "Cool ""identifier""".

However, this escape option was not taken into account by the ADQL library,
though the same mechanism was already in place for string contants.