Fix unused TimeoutStrategy parameter

[martincostello]

The issue or feature being addressed

Fix the timeoutStrategy parameter not being used by Policy.TimeoutAsync<TResult>(Func<TimeSpan>, TimeoutStrategy, Func<Context, TimeSpan, Task, Task>) which meant that TimeoutStrategy.Optimistic was always being used.

I found this issue when experimenting with whether it was worth adding a net5.0 TFM to Polly (TL;DR - I don't think it is) and enabling the new .NET analyzers built into the .NET 5.0 SDK. Doing that flagged up the unused parameter as a CA1801 warning.

Details on the issue fix or feature implementation

Pass the parameter value to Policy.TimeoutAsync<TResult>(Func<Context, TimeSpan>, TimeoutStrategy, Func<Context, TimeSpan, Task, Task>).

I did try to come up with a unit test that verified the wrong strategy was being used pre-fix, but I got stuck with the custom clock implementation in the tests and couldn't get a test using a never-completing callback to not hang forever.

Confirm the following

• I started this PR by branching from the head of the latest dev vX.Y branch, or I have rebased on the latest dev vX.Y branch, or I have merged the latest changes from the dev vX.Y branch
• I have targeted the PR to merge into the latest dev vX.Y branch as the base branch
• I have included unit tests for the issue/feature
• I have successfully run a local build