Default arguments should only be specified in function prototypes, not in the function definition. This caused compilation of the library to fail for Arduino AVR Boards 1.6.11 or earlier. It does not cause compilation to fail for Arduino AVR Boards 1.6.12 and newer only because the -fpermissive compiler flag was added, which downgrades this to an error. It has been stated that flag may be removed in the future:
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So this change is necessary for both backwards and forwards compatibility. Even with current versions it fixes a warning, which is always wise to do.
