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/http.ts

@@ -17,6 +17,7 @@ function mergeOptions(defaultOpts, providedOpts, method, url): RequestOptions {
     newOptions = newOptions.merge(new RequestOptions({
       method: providedOpts.method,
       url: providedOpts.url,
+      search: providedOpts.search,
       headers: providedOpts.headers,
       body: providedOpts.body,
       mode: providedOpts.mode,