fix: add getHeader function for webpack-dev-middleware (#110)

Author: jqueryisamonad

lib/middleware.js

@@ -33,6 +33,7 @@ module.exports = {
               context.body = content;
               resolve();
             },
+            getHeader: context.get.bind(context),
             setHeader: context.set.bind(context),
             locals: context.state
           },