Don't parse the URI unnecessarily.

This becomes more important when using a URI that causes the stdlib `URI` library to freak out, but which `Addressable::URI` can parse e.g.:

```
$ irb
irb(main):001:0> require 'uri'
=> true
irb(main):002:0> URI.parse 'http://foo:^password*12@whatever.com/'
URI::InvalidURIError: bad URI(is not URI?): http://foo:^password*12@whatever.com/
	from /opt/boxen/rbenv/versions/2.1.0/lib/ruby/2.1.0/uri/common.rb:176:in `split'
	from /opt/boxen/rbenv/versions/2.1.0/lib/ruby/2.1.0/uri/common.rb:211:in `parse'
	from /opt/boxen/rbenv/versions/2.1.0/lib/ruby/2.1.0/uri/common.rb:747:in `parse'
	from (irb):2
	from /opt/boxen/rbenv/versions/2.1.0/bin/irb:11:in `<main>'
irb(main):003:0> 
```