Add witness option to Transaction toBuffer()

[benjamin-wilson]

During construction of a coinbase transaction for mining I have to get the serialized coinbase transaction to send to the miners. The miners do not expect the witness data to be in the header. Currently I resort to using the private __toBuffer() method. coinbaseTransaction.__toBuffer().toString('hex'); It would be nice if the toBuffer() and toHex() methods had an option to exclude the witness data.

[jasonandjay]

@benjamin-wilson @ljluestc
Which scenario requires excluding witness data?

[benjamin-wilson]

This is a step in the construction of coinbase transactions for mining