Backport of #3981: Plaintext mode of mailcloak

Hi folks!

PR #3981 fixed the plaintext functionality of the emailcloak plugin for 3.x. This PR fixes the same thing for 2.5.x. 

## Testing instructions
* Edit the plugin "Content - Email Cloaking".
 * Enabled: Yes
 * Mode: Non-linkable text

Use cases of this plugin (every of them with javascript enabled and disabled):
1. ```<a href="mailto:email@example.org">email@example.org</a>```
2. ```<a href="mailto:email@example.org">text</a>```
3. ```<a href="mailto:email@example.org"><img src="..." /></a>```
4. ```<a href="mailto:email@example.org?subject=Text">email@example.org</a>```
5. ```<a href="mailto:email@example.org?subject=Text">text</a>```
6. ```<a href="mailto:email@example.org?subject=Text"><img src="..." /></a>```
7. plain ```email@example.org```