Rework oauth_plaintext to use oauth_util:esprintf.

src/oauth_plaintext.erl

@@ -4,7 +4,5 @@
 
 
 signature(ConsumerSecret, TokenSecret) ->
-  encode(fmt:sprintf("%s&%s", [encode(ConsumerSecret), encode(TokenSecret)])).
-
-encode(String) ->
-  fmt:percent_encode(String).
+  Encoded = oauth_util:esprintf("%s&%s", [ConsumerSecret, TokenSecret]),
+  fmt:percent_encode(Encoded).