feature #6243 Add missing getBoolean() method (bocharsky-bw)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #6243).

Discussion
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Add missing getBoolean() method

According to the [Symfony\Component\HttpFoundation\ParameterBag::getBoolean()](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpFoundation/ParameterBag.php#L187-L190).

Commits
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cb8f241 Add missing getBoolean() method

Author: wouterj

components/http_foundation/introduction.rst

@@ -125,6 +125,9 @@ has some methods to filter the input values:
 :method:`Symfony\\Component\\HttpFoundation\\ParameterBag::getAlnum`
     Returns the alphabetic characters and digits of the parameter value;
 
+:method:`Symfony\\Component\\HttpFoundation\\ParameterBag::getBoolean`
+    Returns the parameter value converted to boolean;
+
 :method:`Symfony\\Component\\HttpFoundation\\ParameterBag::getDigits`
     Returns the digits of the parameter value;