fix(email): Change as_ref() to as_str() to fix nightly bug

[bcmyers]

If you try to build lettre with the latest nightly (2019-05-18), you get the error illustrated below. This is probably not a bug in lettre but rather a bug in the latest Rust nightly. That said, changing all the as_ref calls to as_str calls in lettre_email/src/lib.rs (as this PR does) fixes the issue without really changing the intent of the lettre code.

Not sure this is worth incorporating, as, again, I suspect the problem is really with Rust nightly, not lettre. That being said, this little fix (changing to as_str) ensures lettre continues to work on nighty as of right now.

error[E0283]: type annotations required: cannot resolve `std::string::String: std::convert::AsRef<_>`
   --> lettre_email/src/lib.rs:130:66
    |
130 |         self.header(("Content-Type", format!("{}", content_type).as_ref()))
    |                                                                  ^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0283`.
error: Could not compile `lettre_email`.

To learn more, run the command again with --verbose.

[amousset]

Fixed by removing completely the call to as_ref, thanks for reporting!

[amousset]

Fixed in 0.9.2.