Missing cases from SPI_result_code_string()

Another one noticed from the MERGE RETURNING patch -- the switch
statement in SPI_result_code_string() is missing cases for
SPI_OK_TD_REGISTER and SPI_OK_MERGE.

The SPI_OK_TD_REGISTER case goes back all the way, so I suppose it
should be back-patched to all supported branches, though evidently
this is not something anyone is likely to care about.

The SPI_OK_MERGE case is perhaps a little more visible (e.g., execute
MERGE from PL/Perl using $rv = spi_exec_query() and then examine
$rv->{status}). It's also missing from the docs for SPI_Execute().
Having tested that it now works as expected, I don't think there's
much point in adding a regression test case for it though.

Regards,
Dean