LibIMAP: Don't parse starting space directly in parse_envelope

Makes parse_envelope() look like the defined ABNF rule and the parser
will no longer try to consume double spaces in body-type-msg.

Userland/Libraries/LibIMAP/Parser.cpp

@@ -299,6 +299,7 @@ FetchResponseData Parser::parse_fetch_response()
             break;
         }
         case FetchCommand::DataItemType::Envelope: {
+            consume(" "sv);
             fetch_response.set_envelope(parse_envelope());
             break;
         }