Fix decoding of tab-indented 'charset' of multi-part MIME text parts

Apple Mail apparently uses tabs to indent the charset of nested MIME parts:

e.g.
--Apple-Mail-143B9F0C-6BB4-4C8E-869B-6DE05D6B58CE
Content-Type: text/plain;
<tab>charset=utf-8
Content-Transfer-Encoding: quoted-printable

This fixes issue #483 reported by Nelgin

src/smblib/smbtxt.c

@@ -375,7 +375,10 @@ void smb_parse_content_type(const char* content_type, char** subtype, char** cha
 			}
 		}
 		char* parms = p;
-		if(charset != NULL && ((p = strcasestr(parms, " charset=")) != NULL || (p = strcasestr(parms, ";charset=")) != NULL)) {
+		if(charset != NULL &&
+			((p = strcasestr(parms, " charset=")) != NULL
+				|| (p = strcasestr(parms, ";charset=")) != NULL
+				|| (p = strcasestr(parms, "\tcharset=")) != NULL)) {
 			BOOL quoted = FALSE;
 			p += 9;
 			if(*p == '"') {