fix(Request): Change Request.text's return type to string

Change the return typing for the .text method to `string` so typescript treats it
like a normal string.

Closes #8138

modules/@angular/http/src/static_request.ts

@@ -84,5 +84,5 @@ export class Request {
    * empty
    * string.
    */
-  text(): String { return isPresent(this._body) ? this._body.toString() : ''; }
+  text(): string { return isPresent(this._body) ? this._body.toString() : ''; }
 }