fix(headers): correctly handle repeated headers

HeaderX: a
    HeaderX: b

MUST be interpreted as

    HeaderX: a, b

See: https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2

Fixes #683

src/header/common/cache_control.rs

@@ -1,7 +1,7 @@
 use std::fmt;
 use std::str::FromStr;
 use header::{Header, HeaderFormat};
-use header::parsing::{from_one_comma_delimited, fmt_comma_delimited};
+use header::parsing::{from_comma_delimited, fmt_comma_delimited};
 
 /// `Cache-Control` header, defined in [RFC7234](https://tools.ietf.org/html/rfc7234#section-5.2)
 ///
@@ -55,10 +55,7 @@ impl Header for CacheControl {
     }
 
     fn parse_header(raw: &[Vec<u8>]) -> ::Result<CacheControl> {
-        let directives = raw.iter()
-            .filter_map(|line| from_one_comma_delimited(&line[..]).ok())
-            .collect::<Vec<Vec<CacheDirective>>>()
-            .concat();
+        let directives = try!(from_comma_delimited(raw));
         if !directives.is_empty() {
             Ok(CacheControl(directives))
         } else {