client: don't double-encode the parameters strings

generateParamString() will call URLEncode.encode() on the value when the
scope string is generated later; there's no need to do it twice.

this makes twilio-java behave the same as at least twilio-python and
twilio-ruby, which already do the right thing.
Merge pull request #141 from twilio/fix-captoken-double-encode

client: don't double-encode the parameters strings