Merge pull request #1444 from mikeyhew/docs-basic-fromstr

docs(headers): document the fact that <Basic as FromStr>::from_str takes a base-64 encoded string

src/header/common/authorization.rs

@@ -167,6 +167,7 @@ impl Scheme for Basic {
     }
 }
 
+/// creates a Basic from a base-64 encoded, `:`-delimited utf-8 string
 impl FromStr for Basic {
     type Err = ::Error;
     fn from_str(s: &str) -> ::Result<Basic> {