allow scalar to return custom types

spec/custom_drivers_types_spec.cr

@@ -197,6 +197,13 @@ describe DB do
     end
   end
 
+  it "drivers should return custom values as scalar" do
+    DB.open("foo://host") do |db|
+      FooDriver.fake_row = [FooValue.new(3), FooValue.new(99)] of FooDriver::Any
+      db.scalar("query").as(FooValue).value.should eq(3)
+    end
+  end
+
   it "Foo and Bar drivers should not implement each other read" do
     with_witness do |w|
       DB.open("foo://host") do |db|

src/db/query_methods.cr

@@ -4,7 +4,7 @@ module DB
   #
   # Three kind of statements can be performed:
   #  1. `#exec` waits no record response from the database. An `ExecResult` is returned.
-  #  2. `#scalar` reads a single value of the response. A `DB::Any` is returned.
+  #  2. `#scalar` reads a single value of the response. A union of possible values is returned.
   #  3. `#query` returns a `ResultSet` that allows iteration over the rows in the response and column information.
   #
   # Arguments can be passed by position

src/db/statement.cr

@@ -45,25 +45,7 @@ module DB
     def scalar(*args)
       query(*args) do |rs|
         rs.each do
-          # return case rs.read?(rs.column_type(0)) # :-( Some day...
-          case rs.column_type(0)
-          when String.class
-            return rs.read?(String)
-          when Int32.class
-            return rs.read?(Int32)
-          when Int64.class
-            return rs.read?(Int64)
-          when Float32.class
-            return rs.read?(Float32)
-          when Float64.class
-            return rs.read?(Float64)
-          when Bytes.class
-            return rs.read?(Bytes)
-          when Nil.class
-            return rs.read?(Int32)
-          else
-            raise "not implemented for #{rs.column_type(0)} type"
-          end
+          return rs.read?(rs.column_type(0))
         end
       end