Fix dsn.path where Sentry runs at a non-root URL

with a DSN like https://A:B@host.example.com/sentry/123 ,
parseDSN() produces:

{ protocol: 'https',
  public_key: 'A',
  private_key: 'B',
  host: 'host.example.com',
  path: 'sentry',
  project_id: 123,
  port: 443 }

which means when a request is sent, it goes to sentry/api/store/
instead of /sentry/api/store/.

The change fixes dsn.path to always have a leading (and trailing)
/ and changes the HTTP transport to not append an extra one.

Adds a specific transport test for non-root sentry, and modifies
existing tests with the new behaviour.