Make headers method on WASM client for compatibility with async_impl (#991)

The replace_headers method had to be moved since the async_impl module
is not compiled while compiling to wasm. This caused the replace_headers
method to be unavailable. fast_random had to be excluded from the wasm
build to prevent dead code warnings in the wasm target.

src/util.rs

@@ -1,4 +1,7 @@
+use crate::header::HeaderMap;
+
 // xor-shift
+#[cfg(not(target_arch = "wasm32"))]
 pub(crate) fn fast_random() -> u64 {
     use std::cell::Cell;
     use std::collections::hash_map::RandomState;
@@ -33,3 +36,30 @@ pub(crate) fn fast_random() -> u64 {
         n.0.wrapping_mul(0x2545_f491_4f6c_dd1d)
     })
 }
+
+pub(crate) fn replace_headers(dst: &mut HeaderMap, src: HeaderMap) {
+    // IntoIter of HeaderMap yields (Option<HeaderName>, HeaderValue).
+    // The first time a name is yielded, it will be Some(name), and if
+    // there are more values with the same name, the next yield will be
+    // None.
+    //
+    // TODO: a complex exercise would be to optimize this to only
+    // require 1 hash/lookup of the key, but doing something fancy
+    // with header::Entry...
+
+    let mut prev_name = None;
+    for (key, value) in src {
+        match key {
+            Some(key) => {
+                dst.insert(key.clone(), value);
+                prev_name = Some(key);
+            }
+            None => match prev_name {
+                Some(ref key) => {
+                    dst.append(key.clone(), value);
+                }
+                None => unreachable!("HeaderMap::into_iter yielded None first"),
+            },
+        }
+    }
+}