Fix for precompiled assets when using Proc as asset_host

This fixes the case when ```asset_host``` is used and set to  a Proc to differentiate CDN content from local content.

Scenario: I'm using an ```asset_host``` to serve assets from a CDN, but I want wicked_pdf to still use locally precompiled assets that are stored in their own pdf sub-directories. This is from my production config:
```ruby
  cdn_host = "//s3.amazonaws.com/#{ENV['CDN_BUCKET']}"
  config.action_controller.asset_host = Proc.new { |source|
    # pdf-specific assets are in images/pdf/ and stylesheets/pdf/
    source.include?('/pdf/') ? nil : cdn_host
  }
```

This extends the idea behind issue/pull #119, but because my ```asset_host``` is non-nil, wicked_pdf will incorrectly assume that the ```asset_path``` for my pdf assets is an absolute path. Instead of checking if ```asset_host``` is configured, we should check if ``` asset_path ``` is actually an absolute URL. My fix is something along these lines matching against ``` URI_REGEXP ```.