refactor(headers): errors for parse_header

Header::parse_header() returns now a hyper Result instead of an option
this will enable more precise Error messages in the future, currently
most failures are reported as ::Error::Header.

BREAKING CHANGE: parse_header returns Result instead of Option, related
code did also change

src/header/common/accept_ranges.rs

@@ -52,8 +52,8 @@ pub enum RangeUnit {
 
 
 impl FromStr for RangeUnit {
-    type Err = ();
-    fn from_str(s: &str) -> Result<Self, ()> {
+    type Err = ::Error;
+    fn from_str(s: &str) -> ::Result<Self> {
         match s {
             "bytes" => Ok(RangeUnit::Bytes),
             "none" => Ok(RangeUnit::None),