internal/v4: implement archive uploading

We don't yet verify bundles against charms in the store.

This required some refactoring of the testing code;
the most important change is AssertJSONCall actually
makes a real HTTP call through a socket, as that
allows us to correctly test the ContentLength logic.
It also gives us greater assurance that the logic
will work over a real HTTP connection. The overhead
seems to be negligible.
internal/charmstore: rename *Upload to Add*WithArchive;
internal/v4: use actual upload mechanism for archive get test
internal/v4: make verifyConstraints more obvious
internal/router: add comment to test
Merge pull request #62 from rogpeppe/025-archive-upload

internal/v4: implement archive uploading

We don't yet verify bundles against charms in the store.

This required some refactoring of the testing code;
the most important change is AssertJSONCall actually
makes a real HTTP call through a socket, as that
allows us to correctly test the ContentLength logic.
It also gives us greater assurance that the logic
will work over a real HTTP connection. The overhead
seems to be negligible.