Rework the static method which create the otpauth schema URI. It should not be encoded since it might be used by transports other than the Google Chart QR code generator.
Replace jersey which is a pretty big library if you aren't already using it, with apache httpclient. With all the dependencies this is about half the size and less intrusive for people using other webservice libraries like CXF.

I spent ages trying to find  a more lightweight way to encode URL parameters, but other than extracting the relevant bits of code into this project, there isn't one.
Another test for strange characters in the issuer and username.