Email::context() interface is not clear

[samdark]

Email::context() currently may return two different objects Message and Part. Later return of the method is used in the way that it depends on which object was returned.

• https://github.com/bashkarev/email/blob/master/src/parser/tokens/Content.php#L29
• https://github.com/bashkarev/email/blob/master/src/parser/tokens/Header.php#L55

Overall that is quite confusing...