Fix compile error when target is wasm and multipart feature is enabled.

[Saruniks]

This PR aims to close #1295.

Without this fix master (and v0.11.4) fails to compile with this command:
cargo build --target wasm32-unknown-unknown --features multipart

@seanmonstar Your initial suggestion was to remove #[derive(Clone)] from enum Inner, and change clone() method to try_clone() that matches on the inner type.

But after some research I found out that clone could be implemented for both Inner::Bytes and Inner::Multipart with minimal changes. (I added several #[derive(Clone)] attributes)

What do you think about this change? I am open to implementing your initial suggestion.

I am open to feedback and I will address comments asap.

[Saruniks]

Thanks for the review. I will make fixes asap if the problem is not solved by that time.

[Saruniks]

So far what I managed to do was to fix compile error without exposing clone(). But there's one catch that if Inner type is Inner::Multipart(_) then try_clone returns None.

I have spent some time trying to implement clone to Inner::Multipart(_). But I didn't find a way do it yet without exposing clone() methods to public interface.

[seanmonstar]

What you have now looks right to me, thanks!