Fix the integer width of serial primary keys for 2- and 8-bit integers for Postgres 9.

See https://www.postgresql.org/docs/current/static/datatype-numeric.html.

Author: MrMage

Sources/FluentPostgreSQL/PostgreSQLDatabase+SchemaSupporting.swift

@@ -23,7 +23,12 @@ extension PostgreSQLDatabase: SchemaSupporting, IndexSupporting {
                     string += " GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY"
                 } else {
                     // not appending!
-                    string = "SERIAL PRIMARY KEY"
+                    switch field.type.type {
+                    case .int2: string = "SMALLSERIAL PRIMARY KEY"
+                    case .int4: string = "SERIAL PRIMARY KEY"
+                    case .int8: string = "BIGSERIAL PRIMARY KEY"
+                    default: fatalError("should be unreachable")
+                    }
                 }
             default: string += " PRIMARY KEY"
             }