if available.

(at Georg's request)

Patch by Adreas Stührk.

This makes Header.encode throw a HeaderParseError if it winds up
formatting a header such that a continuation line has no leading
whitespace and looks like a header.  Since Header accepts values
containing newlines and preserves them (and this is by design), without
this fix any program that took user input (say, a subject in a web form)
and passed it to the email package as a header was vulnerable to header
injection attacks.  (As far as we know this has never been exploited.)

Thanks to Jakub Wilk for reporting this vulnerability.

in the operator module.

private by renaming it to `_GeneratorContextManager`.

comparisons that could lead to infinite recursion.

The buildbots will tell us which platform does support or not negative years.

 * With Visual Studio, year have to be in [1; 9999]
 * Add more tests on the year field

Victor Stinner for pointing this out.

This applies only when generating strings from non-RFC compliant binary
input; it makes the existing recoding behavior more consistent (ie:
now no data is lost when recoding).