Revert "Temporary ignored the failing specs for Date"

This reverts commit 17e64cca6b.

The specs should work now.

spec/ruby/library/date/iso8601_spec.rb

@@ -17,23 +17,19 @@ describe "Date.iso8601" do
     d.should == Date.civil(-4712, 1, 1)
   end
 
-  ruby_version_is ""..."3.1" do
   it "parses a Symbol into a Date object" do
     d = Date.iso8601(:'2015-10-15')
     d.should == Date.civil(2015, 10, 15)
   end
-  end
 
   it "parses a StringSubclass into a Date object" do
     d = Date.iso8601(Class.new(String).new("-4712-01-01"))
     d.should == Date.civil(-4712, 1, 1)
   end
 
-  ruby_version_is ""..."3.1" do
   it "raises an ArgumentError when passed a Symbol without a valid Date" do
     -> { Date.iso8601(:test) }.should raise_error(ArgumentError)
   end
-  end
 
   it "raises a TypeError when passed an Object" do
     -> { Date.iso8601(Object.new) }.should raise_error(TypeError)