sql: implicit boolean cast to text returns uppercase

Explicit cast uses uppercase, and the patch makes the
implicit cast the same.

Upper case is the standard according to SQL standard
2011, cast specification 6.13, general rules 11.e:

    General rules

    11) If TD is variable-length character string or
        large object character string, then let MLTD
        be the maximum length in characters of TD.

        e) If SD (source type) is boolean, then Case:

            i)   If SV is True and MLTD is not less
                 than 4, then TV is 'TRUE'.

            ii)  If SV is False and MLTD is not less
                 than 5, then TV is 'FALSE'.

            iii) Otherwise, an exception condition is
                 raised: data exception — invalid
                 character value for cast.

Part of #4462