add TryFrom for AccessControlAllowOrigin

[ghost]

There is currently no way to build an AccessControlAllowOrigin with an actual origin value that isn't Any or Null. This PR adds a TryFrom trait to that header to fix this.

I added a line in the doctest and an additional test to cover this new trait.

It's a bit of an ugly implementation, because the only way to build an OriginOrAny at present is to try_from_values with an iterator.

[ghost]

Wrote a TryFrom for &HeaderValue -> OriginOrAny so there isn't an unnecessary vec! and .iter() in there. Also changed the input type to a &str.

Let me know what you think. @seanmonstar

This partially fixes #48, would need to repeat a similar process across the other headers.

[seanmonstar]

Looks clean to me, thanks!

[ghost]

Thanks! The CI error looks to be that bitflags MSRV is 1.46, while we're building on 1.41. Could we move the MSRV up to 1.46?

[seanmonstar]

Yea, that seems reasonable. hyper is already set to 1.46 as well.

[ghost]

Changed to 1.46 in the Github Action. Didn't see any mentions of MSRV in the docs. Should pass the CI now.

[seanmonstar]

Thanks :)