Update asset manager code written to depend on null collection property defaults

Fixes an error introduced by my change in #2997.
test on travis
test on travis
test on travis
test on travis
Added a getPartial method to the view to return a partial as a string, useful for nested partials.
Per #2397 the file cache now uses a filesystem safe key.
Added the default status codes to the Response class per #2644
Made the request headers return properly
Per #2985 a tiny fix to retrieve the proper height of an image in the GD adapter
Per #2906 had refresh() return the model for fluent interface
Fixed typos in Acl\Adapter\Memory - #2887
Fix ini Config adapter issue
Fix issue #3012
Merge pull request #3009 from rianorie/2.0.0_2887_spellingOfAccess

Fixed typos in Acl\Adapter\Memory - #2887
Merge pull request #3005 from rianorie/2.0.0_2644_defaultStatusCodes

Added the default status codes to the Response class per #2644
Merge pull request #3000 from quantum13/using_last_version_of_composer

Restoring builds on php5.4
Merge pull request #3007 from rianorie/2.0.0_2985_GdCropBug

Per #2985 a tiny fix to retrieve the height of an image in the GD adapter
Merge pull request #3006 from rianorie/2.0.0_2294_MissingCustomHeaders

Made the request headers return properly per #2294
Merge pull request #3008 from rianorie/2.0.0_2906_refreshForFluentInterface

Per #2906 had refresh() return the model for fluent interface