docs($http): describe how to remove a header on a per request basis

Closes #10144

src/ng/http.js

@@ -349,6 +349,21 @@ function $HttpProvider() {
      * In addition, you can supply a `headers` property in the config object passed when
      * calling `$http(config)`, which overrides the defaults without changing them globally.
      *
+     * To explicitly remove a header automatically added via $httpProvider.defaults.headers on a per request basis,
+     * Use the `headers` property, setting the desired header to `undefined`. For example:
+     * 
+     * ```
+     * var req = {
+     *  method: 'POST',
+     *  url: 'http://example.com',
+     *  headers: {
+     *    'Content-Type': undefined
+     *  },
+     *  data: { test: 'test' },
+     * }
+     * 
+     * $http(req).success(function(){...}).error(function(){...});
+     * ```
      *
      * ## Transforming Requests and Responses
      *