Return self from NativeMailerHandler::addHeader().
Return self from NativeMailerHandler::addHeader().

NativeMailerHandler::setContentType() and NativeMailerHandler::setEncoding()
both return the NativeMailerHandler object, allowing a fluent coding
style.  This fix adds the same behaviour to NativeMailerHandler::addHeader()
for so that all public non-inherited methods behave this way for
consistency.

mbrevda/monolog,cherrytech/monolog,aeryaguzov/monolog,esalter/monolog,bd808/monolog,webwlsong/monolog,localheinz/monolog,prgTW/monolog,pomaxa/monolog,deguif/monolog,KevinMGranger/monolog,dailymotion/monolog,rap2hpoutre/monolog,JeroenDeDauw/monolog,yegortokmakov/monolog,tavrij/test,supre/monolog,gencer/monolog,yaoshanliang/monolog,oswaldderiemaecker/monolog,chinaweilu/monolog,dangnguyen27/monolog,brendo/monolog,JCook21/monolog,dracony/monolog,printerpam/monolog,Sgoettschkes/monolog,lyrixx/monolog,thiagotalma/monolog,netgusto/monolog,Seldaek/monolog,entwu/monolog