code: public String getFirst(Object key, boolean percentDecode) {
          SortedSet<String> values = wrappedMap.get(key);
          if (values == null || values.isEmpty()) {
              return null;
          }
          String value = values.first();
          return percentDecode ? OAuth.percentDecode(value) : value;
      }
docs: Returns the first value from the set of all values for the given
  parameter name. If the key passed to this method contains special
  characters, you MUST first percent encode it using
  {@link OAuth#percentEncode(String)}, otherwise the lookup will fail
  (that's because upon storing values in this map, keys get
  percent-encoded).
  
  @param key
  the parameter name (must be percent encoded if it contains unsafe
  characters!)
  @param percentDecode
  whether the value being retrieved should be percent decoded
  @return the first value found for this parameter