clear stashed base url off Drawing module on replot

.. so that in the event where window.location changes on pages
   with a <base>, our clip path urls will be properly updated.

Author: etpinard

src/plot_api/plot_api.js

@@ -113,6 +113,9 @@ exports.plot = function(gd, data, layout, config) {
     // so we can share cached text across tabs
     Drawing.makeTester();
 
+    // clear stashed base url
+    delete Drawing.baseUrl;
+
     // collect promises for any async actions during plotting
     // any part of the plotting code can push to gd._promises, then
     // before we move to the next step, we check that they're all

test/jasmine/tests/plot_interact_test.js

@@ -562,9 +562,6 @@ describe('plot svg clip paths', function() {
     });
 
     it('should set clip path url to ids appended to window url', function(done) {
-        // unstash base url from Drawing module object
-        delete Drawing.baseUrl;
-
         // this case occurs in some past versions of AngularJS
         // https://github.com/angular/angular.js/issues/8934
 
@@ -587,7 +584,6 @@ describe('plot svg clip paths', function() {
             });
 
             base.remove();
-            delete Drawing.baseUrl;
         })
         .catch(failTest)
         .then(done);