RF: has_header made more lenient for up to 6 columns

ukbparse/fileinfo.py

@@ -144,6 +144,8 @@ def has_header(sample,
     # column counts.
     threshold    = collections.defaultdict(lambda : 1)
     threshold[2] = 0
+    threshold[4] = 0
+    threshold[6] = 0
 
     return colcount >= threshold[len(coltypes)]