Trivial fixes.
Make HTTPClientRequest.writeJsonBody use "Content-Length" by default.
Bring vibe.d's enforce closer to std.exception.enforce
Merge pull request #839 from Geod24/enforce-returns

Bring vibe.d's enforce closer to std.exception.enforce
Session opIndexAssign supports the same types as Variant
change cookie info session variables to be compatible with mongoDB key names
sessions support key removal
sessions opIndex now returns a variant for consistency and efficiency
add server option to disable automatic session opening