(MODULES-1771) Don't modify input to is_domain_name()

Fix is_domain_name() so it dup's its incoming argument
to avoid changing the original with a later chomp!

lib/puppet/parser/functions/is_domain_name.rb

@@ -13,7 +13,7 @@ Returns true if the string passed to this function is a syntactically correct do
         "given #{arguments.size} for 1")
     end
 
-    domain = arguments[0]
+    domain = arguments[0].dup
 
     # Limits (rfc1035, 3.1)
     domain_max_length=255

spec/functions/is_domain_name_spec.rb

@@ -61,4 +61,11 @@ describe "the is_domain_name function" do
     result = scope.function_is_domain_name(["not valid"])
     expect(result).to(be_falsey)
   end
+
+  # Values obtained from Facter values will be frozen strings
+  # in newer versions of Facter:
+  it "should not throw an exception if passed a frozen string" do
+    result = scope.function_is_domain_name(["my.domain.name".freeze])
+    expect(result).to(be_truthy)
+  end
 end