Preserve the document URL and resolve relative server URLs #1840

A relative server URL such as /v2, declared at the operation level, overrides the absolute root server but is never resolved against the URL the OpenAPI document was loaded from.

The resulting request URI has no scheme or host, so the call fails before being sent (protocol = null host = null).

Path-level server overrides in OperationDefinition.getServers() have the same problem. The OpenAPI specification allows relative server URLs, which "indicate that the host location is relative to the location where the OpenAPI document is being served" (Server Object). The document URL should be preserved and used to resolve relative server URLs before building the HTTP executor.