Fixed handling absent href attribute in base tag

The HTML5 spec states that the href attribute is optional for the
base tag. Fixed the DomCrawler::addHtmlContent() method to support this

See here and here:
http://www.w3.org/TR/html-markup/base.html#base.attrs.href
http://www.whatwg.org/specs/web-apps/current-work/multipage/semantics.html#the-base-element
[DomCrawler] fix handling of schemes by Link::getUri()

A link (anchor tag with an href attr) in crawled by the Crawler
can contain any valid URI, including mailto: links.

Currently this is not correctly supported by Link::getUri.
Schemes that do not start with 'http' are treated as relative URIs
and appenden to the base URI. This leads to strange URIs like this:
http://foo.com/mailto:foo@bar.com

Fixed Link::getUri to treat any URI with a schema part as an
absolute URL. Updated the unit tests to test for this.
Merge branch '2.1' into 2.2

* 2.1:
  Options small typo
  [Console] fixed unparsed StringInput tokens
  [TwigBridge] fixed trans twig extractor
  [DomCrawler] fix handling of schemes by Link::getUri()
  [Console] Fixed comment
  [TwigBridge] fixed the translator extractor that were not trimming the text in trans tags (closes #7056)
  Fixed handling absent href attribute in base tag
  added a DebuClassLoader::findFile() method to make the wrapping less invasive
  fixed CHANGELOG
  bumped Symfony version to 2.1.9-DEV
  updated VERSION for 2.1.8
  updated CHANGELOG for 2.1.8
  StringInput resets the given options.

Conflicts:
	src/Symfony/Component/HttpKernel/Kernel.php
Merge branch '2.2'

* 2.2: (24 commits)
  Options small typo
  [Console] fixed unparsed StringInput tokens
  Mask PHP_AUTH_PW header in profiler
  [TwigBridge] fixed trans twig extractor
  [Finder] adds adapter selection/unselection capabilities
  [DomCrawler] fix handling of schemes by Link::getUri()
  [Console] Fixed comment
  [TwigBridge] fixed the translator extractor that were not trimming the text in trans tags (closes #7056)
  Fixed handling absent href attribute in base tag
  fixed paths/notPaths regex for shell adapters
  fix issue 4911
  Adds expandable globs support to shell adapters
  [HttpFoundation] Fixed messed up headers
  Fixes AppCache + ESI + Stopwatch problem
  added a DebuClassLoader::findFile() method to make the wrapping less invasive
  bumped Symfony version to 2.2.0-RC4-DEV
  updated VERSION for 2.2.0-RC3
  updated CHANGELOG for 2.2.0-RC3
  fixed CHANGELOG
  bumped Symfony version to 2.1.9-DEV
  ...