New feature: manage encoding/charset

[oroelants]

• in calls: add the possibility to specify templates.contentType property like "text/html;charset=UTF-8" or like "text/plain;charset=UTF-8" (imho, I think it's useful for text/plain and text/html only as other content-type file should enclose the encoding)
• in response: set the charset on header "Content-Type" in coherence with the request.
• eventually check that twig engine handles properly the merge with correct encoding or possible conversion (json data in UTF-8 but template defined in ISO-8859-1)