feat(headers): add AcceptCharset header

Adds support for the Accept-Charset header.  Encodes the charset as a
string.

src/header/common/mod.rs

@@ -8,6 +8,7 @@
 
 pub use self::access_control::*;
 pub use self::accept::Accept;
+pub use self::accept_charset::AcceptCharset;
 pub use self::accept_encoding::AcceptEncoding;
 pub use self::accept_language::AcceptLanguage;
 pub use self::allow::Allow;
@@ -147,6 +148,7 @@ macro_rules! impl_header(
 
 mod access_control;
 mod accept;
+mod accept_charset;
 mod accept_encoding;
 mod accept_language;
 mod allow;