signaturePad.fromData(data) is not setting internal _data variable. (v 2.2.0)

[lpotthast]

Hello,

i do not know if it is intended bahaviour, but using signaturePad.fromData(data) always leaves the _data variable of the SignaturePad object empty.
This results into an empty array being returned if signaturePad.toData() gets called afterwards.

Setting _data manually has fixed the problem for me however.

sig_pad.fromData(data);
sig_pad._data = data;

[szimek]

@inf0rm4tik3r Hey, could you create a PR for this issue?