jsonb_plperl bug

Hi,

I have found a bug in jsonb_plperl extension. A possible fix is proposed below.

jsonb_perl is the contrib module, which defines TRANSFORM functions for jsonb data type and PL/Perl procedural language.

The bug can be reproduced as follows:

CREATE EXTENSION plperl;
CREATE EXTENSION jsonb_plperl;

CREATE OR REPLACE FUNCTION text2jsonb (text) RETURNS jsonb
    LANGUAGE plperl TRANSFORM FOR TYPE jsonb AS
$$
    my $x = shift;
    my $ret = {a=>$x};
    return $ret;
$$;
SELECT text2jsonb(NULL);
SELECT text2jsonb('11');
SELECT text2jsonb(NULL);

The last SELECT produces a strange error.

ERROR:  cannot transform this Perl type to jsonb

A brief research has shown that the problem is in an incomplete logic inside the transform function. The reason can be illustrated by the flollowing Perl one-liner:

perl -MDevel::Peek  -e 'sub x { my $x = shift; Dump $x; warn "----\n\n"; }; x(undef); x("a"); x(undef); '

It outputs:
SV = NULL(0x0) at 0x73a1b8
  REFCNT = 1
  FLAGS = (PADMY)
----

SV = PV(0x71da50) at 0x73a1b8
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x7409a0 "a"\0
  CUR = 1
  LEN = 16
----

SV = PV(0x71da50) at 0x73a1b8
  REFCNT = 1
  FLAGS = (PADMY)
  PV = 0x7409a0 "a"\0
  CUR = 1
  LEN = 16
----

This shows that internal representation of the same undef   in perl is different in first and third function calls. 
It is the way Perl reuses the the lexical variable, probably, for optimization reasons.

Current jsonb_plperl implementation works good for the first (most evident) case, but does not work at all for the third, which results in the abovementioned error.

The attached patch solves this issue and defines corresponding tests.

Regards,
Ivan