It's missing an `)`

Change the links of the logos of different browsers according to the original repository.

Fixes #615

Fix upload progress event example

* Fixing combineURLs to support an empty relativeURL

When combining the base and relative URLs, we should forego force
appending a slash to the base when the relative URL is empty.
This leads to a semantic url.

* Fixing combineURLs, allowing single slash relatives

Removing PROTECTION_PREFIX support

Adding a note that several requests can cancelled with the same token.

I asked the question in #516, pretty sure it will help other people too.

* Adding proxy auth

Default to using "http:" protocol in http adapter

Fixes #490

Fix proxy bugs

Adding support for request cancellation

Fixes #457