devapp: fix travis-ci tests

These tests pass locally (on my Mac) because Go builds in
/etc/apache2/mime.types, which maps the .ico file extension to the
image/x-icon content-type, which we check for in the test.

The Travis-CI builders don't have that file, so they return
vnd.microsoft.icon, which makes the test fail. Instead let's hard code
the content-type that we want, so the test passes.

Verified this failed by moving /etc/apache2/mime.types on my Mac,
observing a failure, then fixing the test and observing a pass.

Change-Id: I42d07fe413d2686d186f9157af8988e3a146a877
Reviewed-on: https://go-review.googlesource.com/38440
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/devapp/noappengine.go b/devapp/noappengine.go
index 6c27e33..822ddc2 100644
--- a/devapp/noappengine.go
+++ b/devapp/noappengine.go
@@ -169,5 +169,6 @@
 }
 
 func faviconHandler(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "image/x-icon")
 	http.ServeFile(w, r, "./static/favicon.ico")
 }