Add staticcheck linter

Fix issues with sync.Pool being passed an array and not a pointer.
See https://github.com/dominikh/go-tools/blob/master/cmd/staticcheck/docs/checks/SA6002

Add missing tests for content.Copy

Fix T.Fatal being called in a goroutine

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

.gometalinter.json

@@ -13,6 +13,7 @@
     "structcheck",
     "unused",
     "varcheck",
+    "staticcheck",
 
     "gofmt",
     "goimports",