Ajax: Support binary data (including FormData)

Two changes have been applied:
* prefilters are now applied before data is converted to a string;
  this allows prefilters to disable such a conversion
* a prefilter for binary data is added; it disables data conversion
  for non-string non-plain-object `data`; for `FormData` bodies, it
  removes manually-set `Content-Type` header - this is required
  as browsers need to append their own boundary to the header

Ref gh-4150
Closes gh-5197

package.json

@@ -55,6 +55,7 @@
     "karma-qunit": "4.1.2",
     "karma-webkit-launcher": "2.1.0",
     "load-grunt-tasks": "5.1.0",
+    "multiparty": "4.2.3",
     "native-promise-only": "0.8.1",
     "playwright-webkit": "1.29.2",
     "promises-aplus-tests": "2.1.2",