Comply exhaustive case check

Comply exhaustive case check

spec/dummy_driver_spec.cr

@@ -246,6 +246,8 @@ describe DummyDriver do
               rs.read(Int64, Int64).should eq({3i64, 4i64})
             when 1
               rs.read(Int64, Int64).should eq({1i64, 2i64})
+            else
+              raise "unreachable"
             end
             i += 1
           end

src/spec.cr

@@ -505,6 +505,8 @@ module DB
                 end
               end
             end
+          else
+            raise "Invalid prepared value. Allowed values are :both and :default"
           end
         end
       end