Fixed behaviour of empty key prefix string

[fohletex]

Hi!

I recently stumpled upon the keyprefix-logic of localforage, when I explicitly didn't want to have a KeyPrefix in my LF instance. Avoiding to define an instance name, will lead to the issue that localforage itself sets the "localforage" prefix. Hence I did

localforage.createInstance({ name: '' });

This replaced the name field with an empty string, however e.g. the LocalStorage driver is still pre-prending a slash (/) to the keys I try to get. As it can be seen here:

localForage/src/drivers/localstorage.js

Line 14 in c1cc34f

var keyPrefix = options.name + '/';

I was wondering if this is a desired behaviour or not. For me it seems to be unwanted and thus I consider it being a bug. This PR will hopefully fix the issue.

I currently further maintain the ReactNative AsyncStorage Driver for LF as well as the WebExtension Storage Driver. If you also see this as a bug and accept the PR or similar I will also re-check these drivers for the same possible issue in these packages and fix if required.

[fohletex]

@tofumatt @thgreasi Any updates? As far as I can see, the build-fail wasn't mine (failed already before the PR). Would be nice if you can give me some further clarification.