fix(history): stricter check of base in HTML5 (#3556)

Co-authored-by: Eduardo San Martin Morote <posva@users.noreply.github.com>

src/history/html5.js

@@ -87,7 +87,13 @@ export class HTML5History extends History {
 
 export function getLocation (base: string): string {
   let path = window.location.pathname
-  if (base && path.toLowerCase().indexOf(base.toLowerCase()) === 0) {
+  const pathLowerCase = path.toLowerCase()
+  const baseLowerCase = base.toLowerCase()
+  // base="/a" shouldn't turn path="/app" into "/a/pp"
+  // https://github.com/vuejs/vue-router/issues/3555
+  // so we ensure the trailing slash in the base
+  if (base && ((pathLowerCase === baseLowerCase) ||
+    (pathLowerCase.indexOf(cleanPath(baseLowerCase + '/')) === 0))) {
     path = path.slice(base.length)
   }
   return (path || '/') + window.location.search + window.location.hash