Change computation of profile ID to follow actual ICC spec

From old spec 4.0.0 (ICC1-V40.pdf, ICC.1:2001-12) to actual one there is a breaking change that makes computation of profile ID different. This commit uses the latest spec 4.4.- The change is the requirement to set header flags to zero instead of attributes when computing the MD5 digest.
Thanks to Timo Kinnunen (@Overruler) for detecting and reporting the issue. This commit should fix #447

src/cmsmd5.c

@@ -251,8 +251,8 @@ void CMSEXPORT cmsMD5finish(cmsProfileID* ProfileID,  cmsHANDLE Handle)
 
 
 // Assuming io points to an ICC profile, compute and store MD5 checksum
-// In the header, rendering intentent, attributes and ID should be set to zero
-// before computing MD5 checksum (per 6.1.13 in ICC spec)
+// In the header, rendering intentent, flags and ID should be set to zero
+// before computing MD5 checksum (per 7.2.18 of ICC spec 4.4)
 
 cmsBool CMSEXPORT cmsMD5computeID(cmsHPROFILE hProfile)
 {
@@ -270,8 +270,8 @@ cmsBool CMSEXPORT cmsMD5computeID(cmsHPROFILE hProfile)
     // Save a copy of the profile header
     memmove(&Keep, Icc, sizeof(_cmsICCPROFILE));
 
-    // Set RI, attributes and ID
-    memset(&Icc ->attributes, 0, sizeof(Icc ->attributes));
+    // Set RI, flags and ID
+    Icc ->flags = 0;
     Icc ->RenderingIntent = 0;
     memset(&Icc ->ProfileID, 0, sizeof(Icc ->ProfileID));