Client: Don't prevent = in secret values provided as args

We're not preventing them when `set key=value` or plain `set key` followed by
an interactively provided value, but the code was blocking them for the specific
`set key=value` where `value` contained a literal `=` sign.

Closes #18.

Client/Program.cs

@@ -350,7 +350,7 @@ void printVersion()
                         secretValue = parts[1];
                         bareArguments.RemoveAt(0);
                     }
-                    else if (bareArguments.Count == 2 && !bareArguments[1].Contains('='))
+                    else if (bareArguments.Count == 2 && !bareArguments[0].Contains('='))
                     {
                         secretName = bareArguments[0];
                         secretValue = bareArguments[1];