pgsql: Fix daterange canonicalization for +/- infinity.

Fix daterange canonicalization for +/- infinity.

The values 'infinity' and '-infinity' are a part of the DATE type
itself, so a bound of the date 'infinity' is not the same as an
unbounded/infinite range. However, it is still wrong to try to
canonicalize such values, because adding or subtracting one has no
effect. Fix by treating 'infinity' and '-infinity' the same as
unbounded ranges for the purposes of canonicalization (but not other
purposes).

Backpatch to all versions because it is inconsistent with the
documented behavior. Note that this could be an incompatibility for
applications relying on the behavior contrary to the documentation.

Author: Laurenz Albe
Reviewed-by: Thomas Munro
Discussion: https://postgr.es/m/77f24ea19ab802bc9bc60ddbb8977ee2d646aec1.camel%40cybertec.at
Backpatch-through: 9.4

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8af1fccba2b1a46d58fad46ed8d03e8c92fe1152

Modified Files
--------------
src/backend/utils/adt/rangetypes.c | 4 ++--
src/test/regress/expected/rangetypes.out | 24 ++++++++++++++++++++++++
src/test/regress/sql/rangetypes.sql | 4 ++++
3 files changed, 30 insertions(+), 2 deletions(-)