Compare FitMethod enum value to correct string

[rvighne]

Previously, the native JNI code compared the FitMethod enum value with the strings "FIT_HERMITE_CUBIC" and "FIT_HERMITE_QUINTIC," however these should be "HERMITE_CUBIC" and "HERMITE_QUINTIC" respectively (to match the names in the Java source code). This caused a bug in which it's impossible to select the quintic method.

I corrected this in the JNI code, not the Java code, so existing users of library don't need to make changes.