fix constness of some char*
fix property enforcement
Add missing imports
Merge pull request #2749 from MartinNowak/curl

fix constness of some char*
Merge pull request #2752 from 9rnsr/fix_imports

Add missing imports
Merge pull request #2751 from 9rnsr/enforceProp

fix property enforcement
isNaN() is not a property

...so it shouldn't be used as one.
std.typecons: remake toString to template, clean module imports

update datetime unittests
Merge pull request #2753 from kyllingstad/isnan-notaproperty

isNaN() is not a property
Merge pull request #2728 from 9il/typecons

std.typecons: remake toString to template, clean module imports
use a Flag!"throwOnError" for Curl.perform
allow to specify throwOnError for high-level wrappers

- in HTTP, FTP, SMTP perform