fix(typings): repair broken type-checking for StringMap

Note that the previous type of StringMap was overly permissive and didn't catch errors.
Also we have to explicitly type empty objects, which is explained here:
https://github.com/Microsoft/TypeScript/issues/5089

Closes #4487

modules/angular2/src/http/headers.ts

@@ -44,7 +44,7 @@ export class Headers {
     if (headers instanceof Headers) {
       this._headersMap = (<Headers>headers)._headersMap;
     } else /*if (headers instanceof StringMap)*/ {
-      this._headersMap = MapWrapper.createFromStringMap<string[]>(headers);
+      this._headersMap = MapWrapper.createFromStringMap<string[]>(<{[key: string]: any}>headers);
       MapWrapper.forEach(this._headersMap, (v, k) => {
         if (!isListLikeIterable(v)) {
           var list = [];