Fix test failing under fresh Sequel by guessing types

It's not clear what changed across c. 50 (!) releases of Sequel but it no longer returns :datetime for columns with altered precision.

This commits fixes the issue by mapping raw DB types to the known symbols. It should not backfire.

lib/rom/sql/schema/type_builder.rb

@@ -37,6 +37,10 @@ def self.[](db_type)
 
         numeric_pk_type Types::Serial
 
+        TYPE_GUESSES = {
+          /datetime\((\d+)\)/ => :datetime
+        }.freeze
+
         def call(primary_key:, db_type:, type:, allow_null:, **rest)
           if primary_key
             map_pk_type(type, db_type, **rest)
@@ -64,7 +68,7 @@ def map_pk_type(_ruby_type, _db_type, **)
 
         # @api private
         def map_type(ruby_type, db_type, **kw)
-          type = self.class.ruby_type_mapping[ruby_type]
+          type = self.class.ruby_type_mapping[ruby_type || guess_type(db_type)]
 
           if (db_type.is_a?(String) && db_type.include?("numeric")) || db_type.include?("decimal")
             map_decimal_type(db_type)
@@ -75,6 +79,15 @@ def map_type(ruby_type, db_type, **kw)
           end
         end
 
+        # @api private
+        def guess_type(db_type)
+          TYPE_GUESSES.find do |regex, type|
+            if db_type.is_a?(String) && db_type.match(regex)
+              break type
+            end
+          end
+        end
+
         # @api private
         def map_decimal_type(type)
           precision = DECIMAL_REGEX.match(type)