Vendor all dependencies (Warning: Huge changeset.)

The vendor/ folder was created with the help of @FiloSottile's gvt and
vendorcheck.

Any dependencies of Caddy plugins outside this repo are not vendored.

We do not remove any unused, vendored packages because vendorcheck -u
only checks using the current build configuration; i.e. packages that
may be imported by files toggled by build tags of other systems.

CI tests have been updated to ignore the vendor/ folder. When Go 1.9 is
released, a few of the go commands should be revised to again use ./...
as it will ignore the vendor folder by default.

caddyhttp/httpserver/server_test.go

@@ -15,7 +15,7 @@ func TestAddress(t *testing.T) {
 	}
 }
 
-func TestMakeHTTPServer(t *testing.T) {
+func TestMakeHTTPServerWithTimeouts(t *testing.T) {
 	for i, tc := range []struct {
 		group    []*SiteConfig
 		expected Timeouts
@@ -111,3 +111,36 @@ func TestMakeHTTPServer(t *testing.T) {
 		}
 	}
 }
+
+func TestMakeHTTPServerWithHeaderLimit(t *testing.T) {
+	for name, c := range map[string]struct {
+		group  []*SiteConfig
+		expect int
+	}{
+		"disable": {
+			group:  []*SiteConfig{{}},
+			expect: 0,
+		},
+		"oneSite": {
+			group: []*SiteConfig{{Limits: Limits{
+				MaxRequestHeaderSize: 100,
+			}}},
+			expect: 100,
+		},
+		"multiSites": {
+			group: []*SiteConfig{
+				{Limits: Limits{MaxRequestHeaderSize: 100}},
+				{Limits: Limits{MaxRequestHeaderSize: 50}},
+			},
+			expect: 50,
+		},
+	} {
+		c := c
+		t.Run(name, func(t *testing.T) {
+			actual := makeHTTPServerWithHeaderLimit(&http.Server{}, c.group)
+			if got := actual.MaxHeaderBytes; got != c.expect {
+				t.Errorf("Expect %d, but got %d", c.expect, got)
+			}
+		})
+	}
+}