acme: expect 202 Accepted from Let's Encrypt

ACME draft specifies the CA servers should respond
with 201 Created status code but Let's Encrypt
responds with 202 Accepted when creating a new account.

This change adds 202 Accepted as a valid response.
Otherwise, the Client hangs while doing retries,
discarding 202 responses as invalid.

Tests are not updated intentionally
due to this being non-conformant with the spec.

Fixes golang/go#26251

Change-Id: I2918fce3873592c02e96f4118c4d1ecb42da3c4f
Reviewed-on: https://go-review.googlesource.com/125415
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

acme/acme.go

@@ -645,8 +645,9 @@ func (c *Client) doReg(ctx context.Context, url string, typ string, acct *Accoun
 		req.Agreement = acct.AgreedTerms
 	}
 	res, err := c.post(ctx, c.Key, url, req, wantStatus(
-		http.StatusOK,      // updates and deletes
-		http.StatusCreated, // new account creation
+		http.StatusOK,       // updates and deletes
+		http.StatusCreated,  // new account creation
+		http.StatusAccepted, // Let's Encrypt divergent implementation
 	))
 	if err != nil {
 		return nil, err