Use "subtle" instead of "encrypt" in Web Cryptography API test

http://www.w3.org/TR/2014/WD-WebCryptoAPI-20140325/#crypto-interface
http://www.w3.org/TR/2014/WD-WebCryptoAPI-20140325/#examples-section
Merge pull request #343 from syoichi/pr3-crypto

Use "subtle" instead of "encrypt" in Web Cryptography API test