Fix incorrectly named ECDSA algorithm

[mike9005]

Per the RFC on ECDSA in TLS, the signing algorithm using the P-521 elliptic curve and SHA-512 is called ES521, instead of ES512.

[coveralls]

Coverage decreased (-21.4%) to 78.598% when pulling ebd2fb9 on mike9005:es521_fix into 3edaa53 on jpadilla:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 89f9a9d on mike9005:es521_fix into 3edaa53 on jpadilla:master.

[jpadilla]

I'm fine with something like this, although we need to do so without breaking compat. We'd need to make sure everything still works for users using ES512 as an algo.

[mark-adams]

I agree with @jpadilla. At least until the next major version, this should still allow ES512 and emit a warning if you use it.