net: handle single-line non-\n-terminated files correctly in readLine

Fixes #6646.

R=rsc, bradfitz
CC=golang-dev
https://golang.org/cl/15960047
diff --git a/src/pkg/net/parse.go b/src/pkg/net/parse.go
index 7c87b42..6056de2 100644
--- a/src/pkg/net/parse.go
+++ b/src/pkg/net/parse.go
@@ -54,7 +54,7 @@
 		if n >= 0 {
 			f.data = f.data[0 : ln+n]
 		}
-		if err == io.EOF {
+		if err == io.EOF || err == io.ErrUnexpectedEOF {
 			f.atEOF = true
 		}
 	}