Fix classes on character count when in error state

The template correctly adds whitespace between classnames when either the `—error` modifier is added or when custom classes are added, but not when both are added.

This means for example that if you were passing the custom class `app-character-count--custom-modifier` to the character count component whilst it was in an error state, the resulting class attribute would be `govuk-js-character-count govuk-textarea--errorapp-character-count--custom-modifier` – neither the error styling nor the custom class would be correctly applied.

Fixes #1630

src/govuk/components/character-count/template.njk

@@ -11,7 +11,7 @@
     rows: params.rows,
     value: params.value,
     formGroup: params.formGroup,
-    classes: 'govuk-js-character-count ' + (' govuk-textarea--error' if params.errorMessage) + (params.classes if params.classes),
+    classes: 'govuk-js-character-count' + (' govuk-textarea--error' if params.errorMessage) + (' ' + params.classes if params.classes),
     label: {
       html: params.label.html,
       text: params.label.text,

src/govuk/components/character-count/template.test.js

@@ -168,6 +168,18 @@ describe('Character count', () => {
       const $component = $('.govuk-js-character-count')
       expect($component.hasClass('govuk-textarea--error')).toBeTruthy()
     })
+
+    it('renders with classes', () => {
+      const $ = render('character-count', {
+        errorMessage: {
+          text: 'Error message'
+        },
+        classes: 'app-character-count--custom-modifier'
+      })
+
+      const $component = $('.govuk-js-character-count')
+      expect($component.hasClass('app-character-count--custom-modifier')).toBeTruthy()
+    })
   })
 
   describe('with dependant components', () => {