Don't start a session if it's known to be empty.

If an app only reads/checks the session there's no need to start a
session to know that the read/checked session value is empty.

Fixes #1981
Avoid code duplication.
correct property comment
Merge pull request #2729 from joostdekeijzer/typo-in-comment

correct property comment
Fix #2721 in TranslateBehavior::beforeFind()  supporting both Model::field('fieldname') and Model::read('fieldname')
Fix Console "extract i18n from sources" to handle quotes in Model::validate parameters correctly
Merge pull request #2728 from joostdekeijzer/2721-fix-in-translatebehavior

Fix Model::field() and  TranslateBehavior::beforeFind() not working together.

Fixes #2721
Merge pull request #2731 from joostdekeijzer/console-extract-not-handling-validation-quotes

Console Extract task doesn't handle validation messages with qoutes correctly
Grammatical fix in API docs only

Grammatical fix in API docs only

reset
Merge pull request #2734 from walker/patch-1

Grammatical fix in API docs only
Actually exclude core test cases from pear packages.

Refs #2620
Merge branch 'master' into 2.5
Added regex support to Validation::mimeType().
Fix link in comment
Merge pull request #2743 from mattalexx/fix-comment

Fix link in comment
Rendering a proper value for html5 attribute 'step'
Using sprintf to convert number into non-scientific notation
Merge pull request #2744 from wnasich/fix_html5_attribute_step

Rendering a proper value for html5 attribute 'step'
Allow empty values in checkboxes.

Allow the checkbox value attribute to be empty. This is required to make
checkboxes with a value of 0.

Fixes #2717
Fix plugin path filtering when directories do not exist.

When paths do not exist we should correctly remove them from the output
paths.

Fixes #2748