[PATCH v1] Allow COPY "text" format to output a header

This patch adds the possibility to use the "header" option when using COPY with
the text format. A todo entry was opened for this and I updated the tests and
the documentation.

This was previously discussed at
https://www.postgresql.org/message-id/flat/CACfv%2BpJ31tesLvncJyP24quo8AE%2BM0GP6p6MEpwPv6yV8%3DsVHQ%40mail.gmail.com

Greetings,
Rémi

---
 doc/src/sgml/ref/copy.sgml          |  3 +-
 src/backend/commands/copy.c         | 11 ++++---
 src/test/regress/input/copy.source  | 46 +++++++++++++++++++----------
 src/test/regress/output/copy.source | 41 +++++++++++++++----------
 4 files changed, 64 insertions(+), 37 deletions(-)