stream: Empty strings/buffers do not signal EOF any longer
stream: read(0) should not always trigger _read(n,cb)

This is causing the CryptoStreams to get into an awful state when
there is a tight loop calling connection.write(chunk) waiting for
a false return.

Because CryptoStreams use read(0) to cycle data, this was causing
the encrypted side to pull way too much data in from the cleartext
side, since the read(0) would make it always call _read.

The unfortunate side effect, fixed in the next patch, is that
CryptoStreams don't automatically cycle when the Socket drains.
tls: Cycle data when underlying socket drains
Fix crypto.hmac behavior with empty keys.

node 0.9.6 introduced Buffer changes that cause the key argument of
Hmac::HmacInit (used in crypto.createHmac) to be NULL when the key is
empty. This argument is passed to OpenSSL's HMAC_Init, which does not
like NULL keys.

This change works around the issue by passing an empty string to
HMAC_Init when the key is empty, and adds crypto.createHmac tests for
the edge cases of empty keys and values.