fix(header): make test_module of header! optional

Closes #490

examples/headers.rs

@@ -0,0 +1,13 @@
+#![deny(warnings)]
+
+#[macro_use]
+// TODO: only import header!, blocked by https://github.com/rust-lang/rust/issues/25003
+extern crate hyper;
+
+// A header in the form of `X-Foo: some random string`
+header! {
+    (Foo, "X-Foo") => [String]
+}
+
+fn main() {
+}