Fix parameter name of IMessage::setHtmlPart according to docComment

[stollr]

Currently the comment and signature of IMessage::setHtmlPart is

    /**
     * [...]
     * @param string $contentTypeCharset the charset to use as the text/html
     *        part's content-type header charset value.
     */
    public function setHtmlPart($resource, $charset = 'UTF-8');

Symfony's DeprecationErrorHandler thinks that in the next main version of the package there will be a new parameter for this function:

Remaining indirect deprecation notices (1)

1x: The "ZBateson\MailMimeParser\Message::setHtmlPart()" method will require a new "string $contentTypeCharset" argument in the next major version of its interface "ZBateson\MailMimeParser\IMessage", not defining it is deprecated.

So I propose to change the parameter's name in the interface according to the name in the php doc.

[stollr]

@zbateson ping

[zbateson]

Thanks @Naitsirch