Generator: improved our replace() method.
Generator: improved our replace() method.

If we can't replace a substring in a given string then we should return that given string rather than an empty string.

hsorby/libcellml,cellml/libcellml,MichaelClerx/libcellml,hsorby/libcellml,nickerso/libcellml,cellml/libcellml,MichaelClerx/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,nickerso/libcellml,hsorby/libcellml,hsorby/libcellml,nickerso/libcellml,MichaelClerx/libcellml