EnvInit middleware

Provide a standard middleware to set c.Env. Don't include it in the
default stack, however, since the RequestID middleware will end up
allocating Env anyways.

Fixes #11

web/middleware/envinit.go

@@ -0,0 +1,26 @@
+package middleware
+
+import (
+	"net/http"
+
+	"github.com/zenazn/goji/web"
+)
+
+// EnvInit is a middleware that allocates an environment map if one does not
+// already exist. This is necessary because Goji does not guarantee that Env is
+// present when running middleware (it avoids forcing the map allocation). Note
+// that other middleware should check Env for nil in order to maximize
+// compatibility (when EnvInit is not used, or when another middleware layer
+// blanks out Env), but for situations in which the user controls the middleware
+// stack and knows EnvInit is present, this middleware can eliminate a lot of
+// boilerplate.
+func EnvInit(c *web.C, h http.Handler) http.Handler {
+	fn := func(w http.ResponseWriter, r *http.Request) {
+		if c.Env == nil {
+			c.Env = make(map[string]interface{})
+		}
+		h.ServeHTTP(w, r)
+	}
+
+	return http.HandlerFunc(fn)
+}