fix(http): convert objects passed to requests into a string (#10124)

This remove a breaking change introduced with commit #e7a8e2757b06d572f614f53b648d2fd75df370d2 where json objects passed to  requests were not converted into string.

BREAKING CHANGE:

The behavior in this commit is the same as before PR 7260 : the objects sent with the request are converted to a string, therefore there is no need for the user to take care of the serialization.

Fixes #10073

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

@@ -128,7 +128,7 @@ export class Request extends Body {
   getBody(): any {
     switch (this.contentType) {
       case ContentType.JSON:
-        return this.json();
+        return this.text();
       case ContentType.FORM:
         return this.text();
       case ContentType.FORM_DATA: