* Fix many version checks to be based on Xcode version instead of iOS version.
* Added/fixed a few expected values according to platform version to match behavior in older macOS versions.
* [Foundation] Set 'sentRequest' when sending a request in NSUrlSessionHandler.
Fixes this compiler warning:
> [..]/external/mono/mcs/class/System.Net.Http/HttpClientEx.cs(50,8): warning CS0649: Field `Foundation.NSUrlSessionHandler.sentRequest' is never assigned to, and will always have its default value `false'
However it changes the runtime behavior, and we'll now throw an exception in
cases that we accepted before:
* `sentRequest` is only read in `EnsureModifiability ()`, which throws an
exception if `sentRequest` is true.
* Previously `sentRequest` was never set (thus the compiler warning), which
meant `EnsureModifiability` would never throw an exception.
* Looking at the similar `CFNetworkHandler` (which has the identical field and
methods), it seems that the intended behavior is to set `sentRequest` in
`SendAsync`, and then `EnsureModifiability` is called whenever a property is
set to ensure the property isn't set too late (and any change would be
ignored because the request was already sent).
* This means that previously setting any property after the request was sent
would not throw any exceptions (even though the change would be ignored),
while with this change we'd start throwing exceptions.
* Add missing tests for the setRequest var.
* Redesign tests to make sure that all handlers run the same code.
* Fix failing test.
* Add the managed handler to the HttpClient tests.
* Fix minor style issues.
Rolf Bjarne Kvinge