Do not set @proxy_uri attributes if nil

With Mechanize (2.7.2) and net-http-persistent (2.9.2) when running:

```
 >> Mechanize.new
```
I get the following exception:

```
NoMethodError: undefined method `user' for nil:NilClass
	from /Users/kenny/.rvm/gems/ruby-1.9.3-head@riffle/gems/net-http-persistent-2.9.2/lib/net/http/persistent.rb:866:in `proxy='
	from /Users/kenny/.rvm/gems/ruby-1.9.3-head@riffle/gems/mechanize-2.7.2/lib/mechanize/http/agent.rb:1189:in `set_proxy'
	from /Users/kenny/.rvm/gems/ruby-1.9.3-head@riffle/gems/mechanize-2.7.2/lib/mechanize.rb:204:in `initialize'
	from (irb):1:in `new'
	from (irb):1
	from /Users/kenny/.rvm/gems/ruby-1.9.3-head@riffle/gems/railties-3.2.16/lib/rails/commands/console.rb:47:in `start'
	from /Users/kenny/.rvm/gems/ruby-1.9.3-head@riffle/gems/railties-3.2.16/lib/rails/commands/console.rb:8:in `start'
	from /Users/kenny/.rvm/gems/ruby-1.9.3-head@riffle/gems/railties-3.2.16/lib/rails/commands.rb:41:in `<top (required)>'
	from script/rails:6:in `require'
	from script/rails:6:in `<main>'
```

The following patch fixes that.