fix wrong link url in rce link plugin

closes CORE-1018

test plan:
- go to an rce
- create two lines of text
- select the first line
- click the link button in the toolbar and make it a link
- select the second line
- click the link button in the toolbar
- it should not be prepopulated with the previous link's url

Change-Id: Iba4e72a3f58bc0372afb71034d520285837973bc
Reviewed-on: https://gerrit.instructure.com/141322
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
QA-Review: Tucker McKnight <tmcknight@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins
Product-Review: Brent Burgoyne <bburgoyne@instructure.com>

public/javascripts/tinymce_plugins/instructure_links/links.js

@@ -321,6 +321,8 @@ import YouTubeApi from './youtube_api'
       $box.find(".disable_inline_content").attr('checked', $a.hasClass('inline_disabled')).triggerHandler('change');
       $box.find(".auto_show_inline_content").attr('checked', $a.hasClass('auto_open')).triggerHandler('change');
       $box.find(".insert_button").text("Update Link");
+    } else {
+      $box.find(".prompt").val('').change();
     }
     $box.dialog({
       width: 425,

spec/coffeescripts/tinymce_plugins/instructure_links/linksSpec.js

@@ -182,3 +182,44 @@ test('it adds in inline_disabled if checked', () => {
   const classes = EditorLinks.buildLinkClasses(priorClasses, box)
   equal(classes, ' inline_disabled')
 })
+
+let renderDialog_ed = null
+
+QUnit.module("InstructureLinks Tinymce Plugin: renderDialog", {
+  setup() {
+    renderDialog_ed = {
+      getBody: () => null,
+      nodeChanged: () => null,
+      selection: {
+        getContent: () => null,
+        getNode: () => ({nodeName: 'SPAN'})
+      }
+    };
+  },
+  teardown() {
+    $("#instructure_link_prompt").remove()
+  }
+})
+
+test("it resets the text field if no existing link is selected", () => {
+  EditorLinks.renderDialog(renderDialog_ed)
+  const $prompt = $("#instructure_link_prompt .prompt")
+  const $btn = $("#instructure_link_prompt .btn")
+  $prompt.val("someurl")
+  $btn.click()
+  EditorLinks.renderDialog(renderDialog_ed)
+  equal($prompt.val(), "")
+})
+
+test("it sets the text field to the href if link is selected", () => {
+  EditorLinks.renderDialog(renderDialog_ed)
+  const $prompt = $("#instructure_link_prompt .prompt")
+  const $btn = $("#instructure_link_prompt .btn")
+  $prompt.val("otherurl")
+  $btn.click()
+  const a = document.createElement('a')
+  a.href = 'linkurl'
+  renderDialog_ed.selection.getNode = () => a
+  EditorLinks.renderDialog(renderDialog_ed)
+  equal($prompt.val(), "linkurl")
+})