Add charset() / headerCharset() methods to CakeEmail class

[suzuki]

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original pull request was #564. change branch to 2.2.

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CakeEmail have many setter / getter methods for mail headers.
But, charset and headerChaset have a only property.

This is a strange. see below :

• now

$email = new CakeEmail;

$email->charset = 'ISO-2022-JP';       // strange here
$email->headerCharset = 'ISO-2022-JP'; // strange here

$email->from('suzuki@example.com');
$email->to('suzuki@example.jp);
$emaul->subject('TEST MAIL');
$email->send('TEST MAIL BODY');

• my patch

$email = new CakeEmail;

$email->charset('ISO-2022-JP');       // this is nice accessor.
$email->headerCharset('ISO-2022-JP'); // this is nice accessor.

$email->from('suzuki@example.com');
$email->to('suzuki@example.jp);
$emaul->subject('TEST MAIL');
$email->send('TEST MAIL BODY');

[lorenzo]

Thanks!