Patch to fix behaviou when char is signed

it seems to me that there is still something wrong:

	1)  the new is_qdtext_char() is incorrect.
	There is a trap if char is implemented as signed char.
	Suppose that c is '\xFF', it will be -1 when converted to an int.
	By definition, c should be qdtext:
	qdtext = <any TEXT except <">>
	TEXT = <any OCTET except CTLs, but including LWS>
	OCTET = <any 8-bit sequence of data>

	2) the character after '\\' could be either part of a quoted-pair
	(together with '\\'), or a normal qdtext, since '\\' itself can
	be treated as a qdtext. This is equivalent to saying that the
	character after '\\' in a quoted string could be ANY octet.

	A patch based on the above two observations is attached.

	Peng
(cherry picked from commit f10730f616bc1ceff05b3987ffc8fcf3492f0f5b)