refactor(headers): Introduce header!() macro, improve documentation

The new macro handles single value headers, list headers, and list
headers with at least one item.
It creates the item for the header and contains its documentation. The
new macro allows handling
more header cases in the future, it will also be possible to include
tests inside the macro.

BREAKING CHANGE: Removed impl_header!() and impl_list_header!() macros,
use new header!() macro.

src/header/common/if_modified_since.rs

@@ -1,9 +1,21 @@
 use header::HttpDate;
 
-/// The `If-Modified-Since` header field.
-#[derive(Copy, PartialEq, Clone, Debug)]
-pub struct IfModifiedSince(pub HttpDate);
-impl_header!(IfModifiedSince, "If-Modified-Since", HttpDate);
+header! {
+    #[doc="`If-Modified-Since` header, defined in"]
+    #[doc="[RFC7232](http://tools.ietf.org/html/rfc7232#section-3.3)"]
+    #[doc=""]
+    #[doc="The `If-Modified-Since` header field makes a GET or HEAD request"]
+    #[doc="method conditional on the selected representation's modification date"]
+    #[doc="being more recent than the date provided in the field-value."]
+    #[doc="Transfer of the selected representation's data is avoided if that"]
+    #[doc="data has not changed."]
+    #[doc=""]
+    #[doc="# ABNF"]
+    #[doc="```plain"]
+    #[doc="If-Unmodified-Since = HTTP-date"]
+    #[doc="```"]
+    (IfModifiedSince, "If-Modified-Since") => [HttpDate]
+}
 
 bench_header!(imf_fixdate, IfModifiedSince, { vec![b"Sun, 07 Nov 1994 08:48:37 GMT".to_vec()] });
 bench_header!(rfc_850, IfModifiedSince, { vec![b"Sunday, 06-Nov-94 08:49:37 GMT".to_vec()] });