Fix suspected bug in PLAINTEXT signature.

This JavaScript implementation:
http://oauth.googlecode.com/svn/code/javascript/oauth.js appears to
indicate that the signature only needs to be encoded once, not twice.
This makes erlang-oauth comply, assuming it is passed an already-decoded
signature to verify (which is usual when using something like Mochiweb,
for example).

Signed-off-by: Tim Fletcher <tim@tfletcher.com>

src/oauth_plaintext.erl

@@ -4,7 +4,7 @@
 
 
 signature(CS, TS) ->
-  oauth_uri:encode(oauth_uri:calate("&", [CS, TS])).
+  oauth_uri:calate("&", [CS, TS]).
 
 verify(Signature, CS, TS) ->
   Signature =:= signature(CS, TS).