Stop Discover's Visualize button from loading incorrect URL

When App state is read from the url, it is accessed via
`$location.search()`. It turns out that the result of
`$location.search()` is a mutable object that allows you to modify the
results of `$location.search()` without actually modifying the URL. When
a user clicks a field in the Discover sidebar, it executes the scope
method `vizLocation` to get the href value for the "Visualize" button.
As an unintended side effect, `vizLocation` was modifying the object
returned from `$location.search()`. This change would ultimately be read
when the Visualize app loaded instead of the param in the actual URL
since there's no full page load from Discover -> Visualize.

To make things more complicated, I believe there was a race condition
partially masking this issue. Since `vizLocation` is used in an Angular
string template, the data binding causes it to execute on every digest
cycle. This is why the incorrect value continued to be written to
`$location.search()` even after `visLocation` ran for the correct field.
It also meant that the result of `$location.search()` was totally
dependent on which field `vizLocation` ran for most recently before the
user clicked Visualize.

Fixes https://github.com/elastic/kibana/issues/8718

src/core_plugins/kibana/public/discover/components/field_chooser/field_chooser.js

@@ -194,7 +194,7 @@ app.directive('discFieldChooser', function ($location, globalState, config, $rou
           };
         }
 
-        return '#/visualize/create?' + $.param(_.assign($location.search(), {
+        return '#/visualize/create?' + $.param(_.assign(_.clone($location.search()), {
           indexPattern: $scope.state.index,
           type: type,
           _a: rison.encode({