feat(headers): Add Pragma header field

Add the HTTP/1.0 `Pragma` header field used to prevent older Caches, that
do not understand the `Cache-Control` header field from caching the ressource.

Closes #237

src/header/common/mod.rs

@@ -23,6 +23,7 @@ pub use self::host::Host;
 pub use self::if_modified_since::IfModifiedSince;
 pub use self::last_modified::LastModified;
 pub use self::location::Location;
+pub use self::pragma::Pragma;
 pub use self::referer::Referer;
 pub use self::server::Server;
 pub use self::set_cookie::SetCookie;
@@ -157,6 +158,7 @@ mod host;
 mod last_modified;
 mod if_modified_since;
 mod location;
+mod pragma;
 mod referer;
 mod server;
 mod set_cookie;
@@ -164,4 +166,3 @@ mod transfer_encoding;
 mod upgrade;
 mod user_agent;
 mod vary;
-