message: fix(ajax): Fix case-insensitive headers in HTTP request
  In RFC 7230 and RFC 7540 is written, that HTTP headers is case-insensitive, so this is fix for correct serializing request body base on HTTP headers.
diff: @@ -202,17 +202,18 @@ export class AjaxSubscriber<T> extends Subscriber<Event> {
  const headers = request.headers = request.headers || {};
  // force CORS if requested
  - if (!request.crossDomain && !headers['X-Requested-With']) {
  + if (!request.crossDomain && !this.getHeader(headers, 'X-Requested-With')) {
  headers['X-Requested-With'] = 'XMLHttpRequest';
  }
  // ensure content type is set
  - if (!('Content-Type' in headers) && !(root.FormData && request.body instanceof root.FormData) && typeof request.body !== 'undefined') {
  + let contentTypeHeader = this.getHeader(headers, 'Content-Type');
  + if (!contentTypeHeader && !(root.FormData && request.body instanceof root.FormData) && typeof request.body !== 'undefined') {
  headers['Content-Type'] = 'application/x-www-form-urlencoded; charset=UTF-8';
  }
  // properly serialize body
  - request.body = this.serializeBody(request.body, request.headers['Content-Type']);
  + request.body = this.serializeBody(request.body, this.getHeader(request.headers, 'Content-Type'));
  this.send();
  }
  @@ -315,6 +316,16 @@ export class AjaxSubscriber<T> extends Subscriber<Event> {
  }
  }
  + private getHeader(headers: {}, headerName: string): any {
  + for (let key in headers) {
  + if (key.toLowerCase() === headerName.toLowerCase()) {
  + return headers[key];
  + }
  + }
  +
  + return undefined;
  + }
  +
  private setupEvents(xhr: XMLHttpRequest, request: AjaxRequest) {
  const progressSubscriber = request.progressSubscriber;
  