[dm-validations] Return unique set of validation errors

 * Original idea was to not take options into account
   in GenericValidator#==, because they often do not matter

 * It did not work out well: when
   you enable inferred length validation for string, but want to                                                                                                                                           later explicitly declare one with Range, both should be added
   or explicit one won't work

 * Given possiblility of annoying duplicates in error messages because
   of "duplicated" validations, we have to "quick fix" it by simply
   returning an unique set

[#671 state:resolved]

dm-validations/lib/dm-validations/validation_errors.rb

@@ -69,7 +69,7 @@ def full_messages
       #   array of validation errors or empty array, if there are no errors on given field
       def on(field_name)
         errors_for_field = errors[field_name]
-        errors_for_field.blank? ? nil : errors_for_field
+        errors_for_field.blank? ? nil : errors_for_field.uniq
       end
 
       def each

dm-validations/lib/dm-validations/validators/generic_validator.rb

@@ -85,10 +85,24 @@ def execute?(target)
         true
       end
 
+      # Returns true if validators are equal
+      #
+      # Note that this intentionally do
+      # validate options equality
+      #
+      # even though it is hard to imagine a situation
+      # when multiple validations will be used
+      # on the same field with the same conditions
+      # but different options,
+      # it happens to be the case every once in a while
+      # with inferred validations for strings/text and
+      # explicitly given validations with different option
+      # (usually as Range vs. max limit for inferred validation)
+      #
+      # @semipublic
       def ==(other)
         self.class == other.class &&
         self.field_name == other.field_name &&
-        self.class == other.class &&
         self.if_clause == other.if_clause &&
         self.unless_clause == other.unless_clause &&
         self.instance_variable_get(:@options) == other.instance_variable_get(:@options)

dm-validations/spec/integration/length_validator/error_message_spec.rb

@@ -5,6 +5,7 @@
 require __dir__.parent.parent + "spec_helper"
 require __dir__ + 'spec_helper'
 
+# FIXME: fixture class(es) need rearrangement
 describe DataMapper::Validate::LengthValidator do
   it "lets user specify custom error message" do
     class Jabberwock

dm-validations/spec/unit/validation_errors/adding_spec.rb

@@ -48,8 +48,8 @@
       @model.should_not be_empty
     end
 
-    it "allows duplication" do
-      @model.on(:property).should == ["can't be valid, no way", "can't be valid, no way"]
+    it "DOES NOT allow duplication" do
+      @model.on(:property).should == ["can't be valid, no way"]
     end
   end  
 end