cleanup: go vet and golint run (#736)

* cleanup: go vet and golint run

Various cleanups trickered by go vet and golint.

* Fix tests and lowercase all errors

Lowercase all errors, some tests in kubernetes use errors from
kubernetes which do start with a capital letter.

middleware/proxy/dns.go

@@ -15,6 +15,7 @@ type dnsEx struct {
 	Options
 }
 
+// Options define the options understood by dns.Exchange.
 type Options struct {
 	ForceTCP bool // If true use TCP for upstream no matter what
 }