[feature/download-etag-fallback] Fallback to Etag (#127)

* - add `authentication.skip-www-authenticate-checks` option to allow force-skipping WWW-Authenticate checks during authentication method detection

* - OCAuthenticationMethodOAuth2: add support for alternative sending of client_id and client_secret in case token endpoint does not support passing them via Basic Auth (RFC 6749, section 2.3.1), new parameter `authentication-oauth2.post-client-id-and-secret`

* - OCConnection: if a download succeeds, but presents no "oc-etag" header, fall back to "Etag" header to determine the file's Etag

ownCloudSDK/Connection/OCConnection.m

@@ -2303,7 +2303,14 @@ - (void)_handleDownloadItemResult:(OCHTTPRequest *)request error:(NSError *)erro
 						file.checksum = [OCChecksum checksumFromHeaderString:checksumString];
 					}
 
-					file.eTag = request.httpResponse.headerFields[@"oc-etag"];
+					OCFileETag eTag = request.httpResponse.headerFields[@"oc-etag"];
+					if (eTag == nil)
+					{
+						// Allow fallback to standard HTTP Etag if oc-etag is not available
+						eTag = request.httpResponse.headerFields[@"Etag"];
+					}
+
+					file.eTag = eTag;
 					file.fileID = file.item.fileID;
 
 					event.file = file;