fix(http): don't encode values that are allowed in query (#9651)

This implements a new class, QueryEncoder, that provides
methods for encoding keys and values of query parameter.
The encoder encodes with encodeURIComponent, and then
decodes a whitelist of allowed characters back to their
unencoded form.

BREAKING CHANGE:

The changes to Http's URLSearchParams serialization now 
prevent encoding of these characters inside query parameters
which were previously converted to percent-encoded values:

@ : $ , ; + ; ? /

The default encoding behavior can be overridden by extending
QueryEncoder, as documented in the URLSearchParams service.

Fixes #9348

modules/@angular/http/http.ts

@@ -34,7 +34,8 @@ export {Http, Jsonp} from './src/http';
 export {Connection, ConnectionBackend, RequestOptionsArgs, ResponseOptionsArgs, XSRFStrategy} from './src/interfaces';
 export {Request} from './src/static_request';
 export {Response} from './src/static_response';
-export {URLSearchParams} from './src/url_search_params';
+export {QueryEncoder, URLSearchParams} from './src/url_search_params';
+
 
 
 /**