Make Sign.getEthereumMessageHash() method public (hyperledger-web3j#1326

)

It's useful for unit testing.

crypto/src/main/java/org/web3j/crypto/Sign.java

@@ -53,7 +53,7 @@ static byte[] getEthereumMessagePrefix(int messageLength) {
         return MESSAGE_PREFIX.concat(String.valueOf(messageLength)).getBytes();
     }
 
-    static byte[] getEthereumMessageHash(byte[] message) {
+    public static byte[] getEthereumMessageHash(byte[] message) {
         byte[] prefix = getEthereumMessagePrefix(message.length);
 
         byte[] result = new byte[prefix.length + message.length];