Clarify client behavior for 502 and 504 response codes

The spec previously said that the client should retry if 502 or 504
response is received. However, it was not specified what retry strategy
should be used if Retry-After header is missing.

The spec now says that the behavior when Retry-After is missing
is the same for all 4 expected codes: 429,502,503,504.

docs/specification.md

@@ -591,8 +591,9 @@ overloaded, the server SHOULD respond with `HTTP 429 Too Many Requests` or
 recommended time interval in seconds to wait before retrying.
 
 The client SHOULD honour the waiting interval specified in the "Retry-After"
-header if it is present. If the client receives an `HTTP 429` or an `HTTP 503`
-response and the "Retry-After" header is not present in the response, then the
+header if it is present. If the client receives a retryable error code
+`HTTP 429`, `HTTP 502`, `HTTP 503` or `HTTP 504`
+and the "Retry-After" header is not present in the response, then the
 client SHOULD implement an exponential backoff strategy between retries.
 
 ##### All Other Responses