EmailConfig: Use CanonicalMIMEHeaderKey instead of TitleCasing for he…

…aders (prometheus#3080)

* EmailConfig: Use CanonicalMIMEHeaderKey instead of TitleCasing for headers

Fixes prometheus#3064.

Signed-off-by: Joe Blubaugh <joe.blubaugh@grafana.com>

* Remove an unused field.

Signed-off-by: Joe Blubaugh <joe.blubaugh@grafana.com>

Signed-off-by: Joe Blubaugh <joe.blubaugh@grafana.com>
Signed-off-by: Yijie Qin <qinyijie@amazon.com>

config/notifiers.go

@@ -15,15 +15,14 @@ package config
 
 import (
 	"fmt"
+	"net/textproto"
 	"regexp"
 	"strings"
 	"time"
 
 	"github.com/pkg/errors"
 	commoncfg "github.com/prometheus/common/config"
 	"github.com/prometheus/common/sigv4"
-	"golang.org/x/text/cases"
-	"golang.org/x/text/language"
 )
 
 var (
@@ -147,8 +146,6 @@ var (
 		Message:              `{{ template "telegram.default.message" . }}`,
 		ParseMode:            "HTML",
 	}
-
-	normalizeTitle = cases.Title(language.AmericanEnglish)
 )
 
 // NotifierConfig contains base options common across all notifier configurations.
@@ -194,7 +191,7 @@ func (c *EmailConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	// Header names are case-insensitive, check for collisions.
 	normalizedHeaders := map[string]string{}
 	for h, v := range c.Headers {
-		normalized := normalizeTitle.String(h)
+		normalized := textproto.CanonicalMIMEHeaderKey(h)
 		if _, ok := normalizedHeaders[normalized]; ok {
 			return fmt.Errorf("duplicate header %q in email config", normalized)
 		}