Dependency injection for jsonld document loader

Some unit tests start clean and create new signer service instance
for every test case. In order for all test cases to reuse a common
caching document loader, it's injected as dependency to the signer.

All this is needed because recently W3C introduced a rate limiter
on their web site and JSONLD context pages, and unit tests fail if
too many requests for resolving contexts are made in short periods
of time.