Don't interpret leading '@'s in POST values.

Curl's default -F switch interprets a leading @ symbol for the value as an instruction to not POST the field/value data, but instead to send the file name that immediately succeeds the symbol (as part of the POST). In order to be able to use passwords starting with an @ symbol, POST data is now being send as application/x-www-form-encoded instead of multipart/form-data whenever a password value starts with an @ symbol. (See eyecatchup/GWT_CrawlErrors-php#8 for further reference.)