Incorrect START TRANSACTION implementation

SQL99 part 2 clause 16.1 contains this note:

NOTE 327 - The characteristics of a transaction begun by a
<start transaction statement> are as specified in these General
Rules regardless of the characteristics specified by any
preceding <set transaction statement>. That is, even if one
or more characteristics are omitted by the <start transaction
statement>, the defaults specified in the Syntax Rules of this
Subclause are effective and are not affected by any (preceding)
<set transaction statement>.

In other words, this should work:

peter=# set session characteristics as transaction read only;
SET
peter=# start transaction;
START TRANSACTION
peter=# create table test (a int);
ERROR: transaction is read-only

because the default of a bare "start transaction" is read write. I
propose that we change this to follow the standard and keep the
traditional behavior for BEGIN only.