Handle invalid ECDSA curve in JWK

The purpose of this pull request is to add a a default switch case for ECDSA curve selection. Previously, if an invalid or empty crv JWK parameter was given for an ECDSA key, there would be a nil pointer dereference panic. You can find more information about valid crv ECDSA JWK parameters in this RFC: https://www.rfc-editor.org/rfc/rfc7518#section-6.2.1.1

Thank you to our new contributor, @btoews!

Related pull requests:

• #66
• #67