`Document#uri` and `Document#getMetadata` returned the base URI rather
than the document's real URL as the document location if the protocol of
the base URI and true document location were different. The reason for
this was to support the use case where the document is an iframe loaded
from a `blob:` URL, which Readium does for example.

This broke in IE 11 if the document did not have a `<base>` element
because the fallback `Node.baseURI` and `Document.documentURI`
properties used by `document-base-uri` do not exist in that browser.

This commit re-works the behavior of `_getDocumentHref` to be more
conservative and return the base URI only if a) the
document location does not have a whitelisted scheme and b) the base URI
is defined and does have a whitelisted scheme.

Additionally, the tests have been re-worked to test the public `uri`
method instead of the private `_getDocumentHref` method and use test
seams instead of proxyquire.