feat(http): serialize search parameters from request options

- Extends URLSearchParams API to include operations for combining
  different URLSearchParams objects:

  These new methods include:
  setAll(otherParams): performs `this.set(key, values[0])` for each
      key/value-list pair in `otherParams`

  appendAll(otherParams): performs `this.append(key, values)` for
      each key/value-list pair in `otherParams`

  replaceAll(otherParams): for each key/value-list pair in
      `otherParams`, replaces current set of values for `key` with
      a copy of the list of values.

- RequestOptions do not merge search params automatically (because
  there are multiple ways to do this). Instead, they replace any
  existing `search` field if `search` is provided. Explicit merging
  is required if merging is desirable.

- Some extra test coverage added.

Closes #2417
Closes #3020

modules/angular2/src/http/interfaces.ts

@@ -12,6 +12,10 @@ import {Headers} from './headers';
 import {BaseException} from 'angular2/src/facade/lang';
 import {EventEmitter} from 'angular2/src/facade/async';
 import {Request} from './static_request';
+import {URLSearchParamsUnionFixer, URLSearchParams} from './url_search_params';
+
+// Work around Dartanalyzer problem :(
+const URLSearchParams_UnionFixer = URLSearchParamsUnionFixer;
 
 /**
  * Abstract class from which real backends are derived.
@@ -41,6 +45,7 @@ export class Connection {
 export interface IRequestOptions {
   url?: string;
   method?: RequestMethods;
+  search?: string | URLSearchParams;
   headers?: Headers;
   // TODO: Support Blob, ArrayBuffer, JSON, URLSearchParams, FormData
   body?: string;