mime/multipart: fix peekBufferSeparatorIndex edge case
The case fixed by this change happens when, in func (pr partReader)
Read, the Peek happens to read so that peek looks like:
"somedata\r\n--Boundary\r"
peekBufferSeparatorIndex was returning (-1, false) because it didn't
find the trailing '\n'.
This was wrong because:
1) It didn't match the documentation: as "\r\n--Boundary" was found, it
should return the index of that pattern, not -1.
2) It lead to an nCopy cut such as:
"somedata\r| |\n--Boundary\r" instead of "somedata| |\r\n--Boundary\r"
which made the subsequent Read miss the boundary, and eventually end
with a "return 0, io.ErrUnexpectedEOF" case, as reported in: