Added missing method Cursor.select_statement?

[rammpeter]

This method was missed for the JDBC implementation of class Cursor

Fixes issue #2470

[jlahtinen]

Seems to be working.

I noticed there are fixes in release80 branch but there is no release done that contains those fixes. Is it clear to someone how/when they are released? Will the release contain this fix? What about the currently unnecessary # Workaround for jruby/jruby#6267 etc

[yahonda]

For awareness: JDBCConnection::Cursor#select_statement? has already been added on master in 0929019b with a simpler implementation:

def select_statement?
  !@raw_result_set.nil?
end

On master this works because an earlier refactor (4d7e9fe8 — "Use createStatement for DDL / PL/SQL in JDBC prepare") changed Cursor#exec to assign @raw_result_set only when @raw_statement.execute returns true (i.e. the first result is a ResultSet), which is JDBC's authoritative SELECT signal. On release80, exec still calls executeQuery unconditionally, so the same one-liner wouldn't distinguish SELECT from DML there — which is why this PR's regex-on-SQL workaround is needed on that branch.

So master doesn't need this change. Whether to merge this PR as-is on release80, or to backport 4d7e9fe8 + 0929019b instead, is a separate release80-only decision.