refactor(header): Implement HttpDate, a wrapper for dates

Using `time::Tm` directly in HTTP header fields requires special handling to parse and format
the header values., this stops us from using the header macros. By wrapping `time::Time` in a
`HttpDate`, we can use the `FromStr` and `Display` traits of `HttpDate` like for most other values.

BREAKING_CHANGE: All code using one of the `Date`, `Expires`, `If-Modified-Since`,
`If-Unmodified-Since`, `Last-Modified` header fields needs to wrap `time::Tm`
with `HttpDate`. Removed `FromStr` trait of `Date`, `If-Modified-Sice` and `If-Unmodified-Sice`,
implementing the trait here is inconsistent with other headers.

src/header/shared/mod.rs

@@ -1,9 +1,11 @@
 pub use self::charset::Charset;
 pub use self::encoding::Encoding;
 pub use self::entity::EntityTag;
+pub use self::httpdate::HttpDate;
 pub use self::quality_item::{Quality, QualityItem, qitem, q};
 
 mod charset;
 mod encoding;
 mod entity;
+mod httpdate;
 mod quality_item;