Update return type for Model::saveField().

Fixes #5086
Fix HttpSocket mishandling encoded URIs

The HTTP specs seem to indicate that the Location header should contain
an 'absoluteURI' which includes encoded data. In order to avoid
a regression with the issue fixed in b9ee4fc9f1abd1639bc20a5536f87ad7a5318dea
we'll continue to replace `%2F` and no longer decode the entire URI.

Fixes #5076
Merge pull request #5096 from cakephp/issue-5076

Fix HttpSocket mishandling encoded URIs
Merge branch 'master' into 2.6
Merge branch '2.6' of github.com:cakephp/cakephp into 2.6