BUG #14920: TEXT binding not works correctly with BPCHAR

The following bug has been logged on the website:

Bug reference:      14920
Logged by:          Jorge Solorzano
Email address:      jorsol@gmail.com
PostgreSQL version: 10.1
Operating system:   Ubuntu 16.04
Description:

TEXT type is the preferred data type for the category String, but I'm having
a hard time using it with bpchar:

The reproducible use case is here:
1. Create a table with a bpchar(3).
2. Insert a tuple with trailing space.
3. If I select the values using varchar, it works fine with and without
trailing space.
4. If I select the values using text, it only works without trailing
space.

This behavior is odd and is not documented (at least I not found it). This
can be a limiting factor to use the text data type for strings instead of
varchar.

------------------------------------------

DROP TABLE IF EXISTS texttable;
CREATE TABLE texttable (
 ch bpchar(3), te text, vc varchar(3)
);

PREPARE fooplaninsert (text, text, text) AS
    INSERT INTO texttable (ch, te, vc) VALUES ($1, $2, $3);
EXECUTE fooplaninsert('c  ', 'c  ', 'c  ');

-- USING VARCHAR
PREPARE fooplanselectvc (varchar, varchar, varchar) AS
    SELECT ch, te, vc FROM texttable WHERE ch=$1 AND te=$2 AND vc=$3;
-- THIS WORKS WITH AND WITHOUT TRAILING SPACE
EXECUTE fooplanselectvc('c  ', 'c  ', 'c  ');
EXECUTE fooplanselectvc('c', 'c  ', 'c  ');

-- USING TEXT
PREPARE fooplanselecttx (text, text, text) AS
    SELECT ch, te, vc FROM texttable WHERE ch=$1 AND te=$2 AND vc=$3;
-- THIS NOT WORKS WITH TRAILING SPACE
EXECUTE fooplanselecttx('c  ', 'c  ', 'c  ');
-- THIS WORKS WITHOUT TRAILING SPACE
EXECUTE fooplanselecttx('c', 'c  ', 'c  ');