Implement changes suggested by @seanmonstar

https://github.com/hyperium/http/pull/326/files#r302696965
https://github.com/hyperium/http/pull/326/files#r302697620

Author: lovasoa

src/convert.rs

@@ -1,5 +1,5 @@
 use Error;
-use header::{HeaderName, HeaderValue};
+use header::{HeaderName, HeaderValue, HeaderMap};
 use method::Method;
 use sealed::Sealed;
 use status::StatusCode;
@@ -56,6 +56,7 @@ reflexive! {
     Uri,
     Method,
     StatusCode,
+    HeaderMap,
     HeaderName,
     HeaderValue,
     Scheme,

src/header/map.rs

@@ -1,15 +1,16 @@
-use super::HeaderValue;
-use super::name::{HeaderName, HdrName, InvalidHeaderName};
-use convert::{HttpTryFrom, HttpTryInto};
-use Error;
-use sealed::Sealed;
-
 use std::{fmt, mem, ops, ptr, vec};
+use std::collections::{BTreeMap, HashMap};
 use std::collections::hash_map::RandomState;
-use std::hash::{BuildHasher, Hasher, Hash};
+use std::hash::{BuildHasher, Hash, Hasher};
 use std::iter::FromIterator;
 use std::marker::PhantomData;
 
+use convert::{HttpTryFrom, HttpTryInto};
+use Error;
+
+use super::HeaderValue;
+use super::name::{HdrName, HeaderName, InvalidHeaderName};
+
 pub use self::as_header_name::AsHeaderName;
 pub use self::into_header_name::IntoHeaderName;
 
@@ -1724,7 +1725,23 @@ impl<T> FromIterator<(HeaderName, T)> for HeaderMap<T>
     }
 }
 
-impl<T> Sealed for HeaderMap<T> {}
+/// A marker trait that allows conversion from a type to HeaderMap
+/// This trait is needed to have both a fast implementation of
+/// HttpTryFrom<HeaderMap> and a generic implementation
+/// of HttpTryFrom<IntoIterator<Item=(K,V)>> for HeaderMap that do not conflict.
+pub trait IntoHeaderMapAllowed {}
+
+impl<K, V, S> IntoHeaderMapAllowed for HashMap<K, V, S> {}
+
+impl<'a, K, V, S> IntoHeaderMapAllowed for &'a HashMap<K, V, S> {}
+
+impl<K, V> IntoHeaderMapAllowed for BTreeMap<K, V> {}
+
+impl<'a, K, V> IntoHeaderMapAllowed for &'a BTreeMap<K, V> {}
+
+impl<'a, T> IntoHeaderMapAllowed for &'a [T] {}
+
+impl<T> IntoHeaderMapAllowed for Vec<T> {}
 
 /// Convert a collection of tuples into a HeaderMap
 ///
@@ -1746,7 +1763,7 @@ impl<T> Sealed for HeaderMap<T> {}
 /// ```
 impl<C, K, V> HttpTryFrom<C> for HeaderMap<HeaderValue>
     where
-        C: IntoIterator<Item=(K, V)>,
+        C: IntoIterator<Item=(K, V)> + IntoHeaderMapAllowed,
         HeaderName: HttpTryFrom<K>,
         HeaderValue: HttpTryFrom<V>
 {
@@ -3132,7 +3149,7 @@ mod into_header_name {
     use super::{HdrName, HeaderMap, HeaderName};
 
     /// A marker trait used to identify values that can be used as insert keys
-    /// to a `HeaderMap`.
+                                            /// to a `HeaderMap`.
     pub trait IntoHeaderName: Sealed {}
 
     // All methods are on this pub(super) trait, instead of `IntoHeaderName`,
@@ -3204,7 +3221,7 @@ mod as_header_name {
     use super::{Entry, HdrName, HeaderMap, HeaderName, InvalidHeaderName};
 
     /// A marker trait used to identify values that can be used as search keys
-    /// to a `HeaderMap`.
+                                            /// to a `HeaderMap`.
     pub trait AsHeaderName: Sealed {}
 
     // All methods are on this pub(super) trait, instead of `AsHeaderName`,