vaadin-flow-bundle-manifest.json is not found when its context path has double slash

[SomeoneToIgnore]

Based on https://vaadin.slack.com/archives/C6X43FE8M/p1518702014000461

When a path to bundle manifest resolves in something like ‘/frontend-es6//vaadin-flow-bundle-manifest.json’ , the code in

flow/flow-server/src/main/java/com/vaadin/flow/server/startup/BundleFilterInitializer.java

Line 95 in edc068a

try (InputStream bundleManifestStream = servletContext

fails to get the resource by this path.
When double slash is removed, it works correctly.

Needs to be fixed to eliminate double slashes.