This commit updates `acme/http.go` to allow customizing the
`*x509.CertPool` used by the `HTTPClient` by specifying the filepath of
a custom CA certificate via the `CA_CERTIFICATE` environment variable.
This allows developers to easily trust a non-standard CA when
interacting with an ACME test server (e.g. Pebble):
```
CA_CERTIFICATE=~/go/src/github.com/letsencrypt/pebble/test/certs/pebble.minica.pem \
lego \
--server https://localhost:14000/dir \
--email foo@bar.com \
-d example.com \
run
```
* Move nonce retry from jws to http
The error raised by an "invalid nonce" response never appeared
inside jws.go, but instead it was handled at http.go, so it makes
sense to move the retry logic to that file. The previous code from
jws.go had no effect and did not solve issues related to invalid
nonces.
* Rename retry response variable name for clarity
This will prevent indefinitely-hanging requests in case some service or middle box is malfunctioning.
Fix vet errors and lint warnings
Add vet to CI check
Only get issuer certificate if it would be used
No need to make a GET request if the OCSP server is not specified in leaf certificate
Fix CI tests
Make tests verbose
