code: public static String encodeStringURL(byte[] source, int pos, int limit, boolean wrap) {
          return Encoder.encodeString(source, pos, limit, wrap, true);
      }
docs: Encodes a fixed and complete byte array into a Base64url String.
  
  <p>This method is only useful for applications which require a String and have all data to be encoded up-front.
  Note that byte arrays or buffers are almost always a better storage choice. They consume half the memory and
  can be reused (modified). In other words, it is almost always better to use {@link #encodeBytes},
  {@link #createEncoder}, or {@link #createEncoderOutputStream} instead.</p>
  
  <pre><code>
  // Encodes "ell"
  FlexBase64.encodeStringURL("hello".getBytes("US-ASCII"), 1, 4);
  </code></pre>
  
  @param source the byte array to encode from
  @param pos the position to start encoding from
  @param limit the position to halt encoding at (exclusive)
  @param wrap whether or not to wrap the output at 76 chars with CRLFs
  @return a new String representing the Base64url output