Dimitri Fontaine d8b0bd5145 Allow casting rules to guard on signed data types.
It used to be that our casting rules mechanism would allow for matching
unsigned data types only, and we sometimes have a need to do special
behavior on signed data types.

In particular, a signed bigint(20) in MySQL has the same values range as a
PostgreSQL bigint, so we don't need to target a numeric in that case. It's
only when the bigint is unsigned that we need to target a numeric.

In passing update some of the default casting rules documentation to match
the code.

Fix #982.
2019-06-04 15:22:25 +02:00
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