Fix missing space in label_attr

``label_attr`` in ``checkbox_radio_label`` is missing a space with ``parent_label_class``
Merge branch '2.6' into 2.7

* 2.6: (24 commits)
  [Form] fixed a maxlength overring on a guessing
  [Debug] Show only unique class candidates
  [SecurityBundle] Firewall providers building - code cleaning
  [Filesystem] symlink use RealPath instead LinkTarget
  [DependencyInjection] Remove duplicate  declaration in PhpDumper
  terminals are not interactive on Travis
  Revert "[DependencyInjection] backport perf optim"
  [WebProfiler] Tweaked ajax requests toolbar css reset
  [WebProfilerBundle] replaced pattern to path attribute in routes definitions.
  fix phpdoc's alignment
  Fix missing addExpressionLanguageProvider (used by service container to add expression providers)
  Fixed the AuthenticationProviderInterface alignment
  Fixed the proxy-manager version constraint
  Fix missing space in label_attr
  fix DumpDataCollectorTest after CS changes
  avoid risky tests
  Fixed typo in SecurityContext PHPDoc
  [FrameworkBundle][Template name] avoid  error message for the shortcut notation.
  [DependencyInjection] perf optim: call dirname() at most 5x
  [DependencyInjection] backport perf optim
  ...