Ignore required rules when group specification has group option set.

Author: hadzhiyski

src/CommandLine/Core/SpecificationPropertyRules.cs

@@ -101,6 +101,7 @@ where sp.Specification.Required
                                            where sp.Value.IsNothing()
                                            let o = (OptionSpecification)sp.Specification
                                            where o.SetName.Length > 0
+                                           where o.Group.IsNothing()
                                            where setWithRequiredValue.ContainsIfNotEmpty(o.SetName)
                                            select sp.Specification;
                 var missing =
@@ -113,6 +114,7 @@ where sp.Specification.Required
                             where sp.Value.IsNothing()
                             let o = (OptionSpecification)sp.Specification
                             where o.SetName.Length == 0
+                            where o.Group.IsNothing()
                             select sp.Specification)
                         .Concat(
                             from sp in specProps

src/CommandLine/OptionAttribute.cs

@@ -107,7 +107,7 @@ public char Separator
         }
 
         /// <summary>
-        /// Gets or sets the option group name. When one or more options are grouped, at least one of them should have value.
+        /// Gets or sets the option group name. When one or more options are grouped, at least one of them should have value. Required rules are ignored.
         /// </summary>
         public string Group
         {