Add custom converter spec detailed as example in the manual

manual/model/column-types/converters.md

@@ -4,6 +4,8 @@ Any type from PostgreSQL can be converted using converter objects. By default, C
 
 However, custom type may not be supported yet. Clear offers you the possibility to add a custom converter.
 
+The example demonstrated below is also available in `spec/model/converters/custom_converter_spec`
+
 ## Declare a new converter
 
 The example below with a converter for a `Color` structure should be straight-forward:

spec/model/converters/custom_converter_spec.cr

@@ -0,0 +1,66 @@
+struct MyApp::Color
+  property r : UInt8 = 0_u8
+  property g : UInt8 = 0_u8
+  property b : UInt8 = 0_u8
+  property a : UInt8 = 0_u8
+
+  def to_s
+    @r + @g + @b + @a
+  end
+
+  def initialize(@r, @g, @b, @a)
+  end
+
+  def self.from_string(x : String)
+    raise ArgumentError.new("invalid size") if x.size != 8
+    array = x.split("")
+    self.new((array[0] + array[1]).to_u8, (array[2] + array[3]).to_u8, (array[4] + array[5]).to_u8, (array[6] + array[7]).to_u8)
+  end
+
+  def self.from_slice(x : Slice(UInt8))
+    raise ArgumentError.new("invalid size") if x.size != 4
+    self.new(x[0], x[1], x[2], x[3])
+  end
+end
+
+class MyApp::ColorConverter
+  def self.to_column(x) : MyApp::Color?
+    case x
+    when Nil
+      nil
+    when Slice(UInt8)
+      MyApp::Color.from_slice(x)
+    when String
+      MyApp::Color.from_string(x)
+    else
+      raise "Cannot convert from #{x.class} to MyApp::Color"
+    end
+  end
+
+  def self.to_db(x : MyApp::Color?)
+    x.to_s # < css style output, e.g. #12345400
+  end
+end
+
+Clear::Model::Converter.add_converter("MyApp::Color", MyApp::ColorConverter)
+
+class MyApp::MyModel
+  include Clear::Model
+  column color : MyApp::Color # < Automatically get the converter
+end
+
+describe "Clear::Model::Converter" do
+  it "should create a new model with a field from a new converter with a new type" do
+    mdl1 = MyApp::MyModel.new({color: "12345400"})
+    mdl1.color.r.should eq(12)
+    mdl1.color.g.should eq(34)
+    mdl1.color.b.should eq(54)
+    mdl1.color.a.should eq(0)
+
+    mdl2 = MyApp::MyModel.new({color: Slice[12_u8, 34_u8, 54_u8, 0_u8]})
+    mdl2.color.r.should eq(12)
+    mdl2.color.g.should eq(34)
+    mdl2.color.b.should eq(54)
+    mdl2.color.a.should eq(0)
+  end
+end