Fix unsetting deprecation_reason

Author: rmosolgo

lib/graphql/schema/member/has_deprecation_reason.rb

@@ -5,20 +5,16 @@ class Schema
     class Member
       module HasDeprecationReason
         # @return [String, nil] Explains why this member was deprecated (if present, this will be marked deprecated in introspection)
-        def deprecation_reason
-          @deprecation_reason
-        end
+        attr_reader :deprecation_reason
 
         # Set the deprecation reason for this member, or remove it by assigning `nil`
         # @param text [String, nil]
         def deprecation_reason=(text)
+          @deprecation_reason = text
           if text.nil?
             remove_directive(GraphQL::Schema::Directive::Deprecated)
           else
-            if defined?(@deprecation_reason) && @deprecation_reason
-              remove_directive(GraphQL::Schema::Directive::Deprecated)
-            end
-            @deprecation_reason = text
+            # This removes a previously-attached directive, if there is one:
             directive(GraphQL::Schema::Directive::Deprecated, reason: text)
           end
         end

spec/graphql/schema/argument_spec.rb

@@ -349,6 +349,13 @@ def self.object_from_id(id, ctx)
     it "has an assignment method" do
       arg.deprecation_reason = "another new deprecation reason"
       assert_equal "another new deprecation reason", arg.deprecation_reason
+      assert_equal 1, arg.directives.size
+      arg.deprecation_reason = "something else"
+      assert_equal "something else", arg.deprecation_reason
+      assert_equal 1, arg.directives.size
+      arg.deprecation_reason = nil
+      assert_nil arg.deprecation_reason
+      assert_equal 0, arg.directives.size
     end
 
     it "disallows deprecating required arguments in the constructor" do