fix: QPCleaner line length equal sign counting

[pavelbazika]

Hi,

as mentioned in #254, this fixes missing counting of = when quoted printable encoded char is processed.

I've also mentioned, that line length is not incremented, when qp.overflow is put into the buffer at the beginning of the next Read. Tests show it is not needed, because qp.lineLength is set, when qp.overflow is filled at the end of previous Read.

Regards

[jhillyerd]

Thank you!