Add specs for #dig with no arguments

Author: ndnenkov

core/array/dig_spec.rb

@@ -33,7 +33,7 @@
       }.should raise_error(TypeError)
     end
 
-    it "raises without any args" do
+    it "raises an ArgumentError if no arguments provided" do
       lambda {
         [10].dig()
       }.should raise_error(ArgumentError)

core/hash/dig_spec.rb

@@ -28,7 +28,7 @@
       h.dig(:foo, :zot, :xyz).should == nil
     end
 
-    it "raises without args" do
+    it "raises an ArgumentError if no arguments provided" do
       lambda { { the: 'borg' }.dig() }.should raise_error(ArgumentError)
     end
 

core/struct/dig_spec.rb

@@ -25,6 +25,10 @@
     }.should raise_error(TypeError)
   end
 
+  it "raises an ArgumentError if no arguments provided" do
+    lambda { @instance.dig }.should raise_error(ArgumentError)
+  end
+
   it "calls #dig on any intermediate step with the rest of the sequence as arguments" do
     obj = Object.new
     instance = @klass.new(obj)