Fix GH-820. CryptoStream.end shouldn't throw if not writable

This matches the behavior of net.Socket

lib/tls.js

@@ -168,10 +168,7 @@ CryptoStream.prototype.getCipher = function(err) {
 
 
 CryptoStream.prototype.end = function(d) {
-  if (!this.writable) {
-    throw new Error('CryptoStream is not writable');
-  }
-
+  if (this.writable) {
     if (this.pair._done) return;
 
     if (d) {
@@ -187,6 +184,7 @@ CryptoStream.prototype.end = function(d) {
     this.writable = false;
 
     this.pair._cycle();
+  }
 };