Fix handling of invalid encoding with nil log.

Fixes: #469

lib/restclient/response.rb

@@ -56,7 +56,9 @@ module RestClient
       begin
         encoding = Encoding.find(charset) if charset
       rescue ArgumentError
-        RestClient.log << "No such encoding: #{charset.inspect}"
+        if RestClient.log
+          RestClient.log << "No such encoding: #{charset.inspect}"
+        end
       end
 
       return unless encoding

spec/integration/integration_spec.rb

@@ -98,6 +98,16 @@ describe RestClient do
       response.encoding.should eq Encoding.default_external
     end
 
+    it 'handles invalid encoding' do
+      stub_request(:get, 'www.example.com').to_return(
+        body: 'abc', status: 200, headers: {
+          'Content-Type' => 'text; charset=plain'
+        })
+
+      response = RestClient.get 'www.example.com'
+      response.encoding.should eq Encoding.default_external
+    end
+
     it 'leaves images as binary' do
       gif = Base64.strict_decode64('R0lGODlhAQABAAAAADs=')