more consistent IDNA handling of wrong input

lib/addressable/idna.rb

@@ -32,13 +32,13 @@ class << self
 
       # public interface implemented by all backends
       def to_ascii(value)
-        backend.to_ascii(value)
+        backend.to_ascii(value) if value.is_a?(String)
       rescue Error
         strict_mode ? raise : value
       end
 
       def to_unicode(value)
-        backend.to_unicode(value)
+        backend.to_unicode(value) if value.is_a?(String)
       rescue Error
         strict_mode ? raise : value
       end

spec/addressable/idna_spec.rb

@@ -152,6 +152,13 @@
       "example..host"
     )).to eq("example..host")
   end
+
+  it "handles nil input" do
+    expect(Addressable::IDNA.to_ascii(nil)).to eq(nil)
+    expect(Addressable::IDNA.to_ascii(45)).to eq(nil)
+    expect(Addressable::IDNA.to_ascii([])).to eq(nil)
+    expect(Addressable::IDNA.to_ascii({})).to eq(nil)
+  end
 end
 
 shared_examples_for "converting from ASCII to unicode" do
@@ -256,6 +263,13 @@
       "example..host"
     )).to eq("example..host")
   end
+
+  it "handles unexpected input as nil" do
+    expect(Addressable::IDNA.to_unicode(nil)).to eq(nil)
+    expect(Addressable::IDNA.to_unicode(45)).to eq(nil)
+    expect(Addressable::IDNA.to_unicode([])).to eq(nil)
+    expect(Addressable::IDNA.to_unicode({})).to eq(nil)
+  end
 end
 
 describe Addressable::IDNA, "when using the pure-Ruby implementation" do